Composition and method for making a proppant

ABSTRACT

The present invention relates to proppants which can be used to prop open subterranean formation fractions. Proppant formulations are further disclosed which use one or more proppants of the present invention. Methods to prop open subterranean formation fractions are further disclosed. In addition, other uses for the proppants of the present invention are further disclosed, as well as methods of making the proppants.

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/347,664, filed Feb. 3, 2006, which in turn claims thebenefit under 35 U.S.C. §119(e) of prior U.S. Provisional PatentApplication No. 60/649,594 filed Feb. 4, 2005, both of which areincorporated in their entirety by reference herein.

BACKGROUND OF THE INVENTION

Recognition that the macroscopic properties of materials depend not onlyon their chemical composition, but also on the size, shape and structurehas spawned investigations into the control of these parameters forvarious materials. In this regard, the fabrication of uniform hollowspheres has recently gained much interest. Hollow capsules withnanometer and micrometer dimensions offer a diverse range of potentialapplications, including utilization as encapsulants for the controlledrelease of a variety of substances, such as drugs, dyes, proteins, andcosmetics. When used as fillers for coatings, composites, insulatingmaterials or pigments, hollow spheres provide advantages over thetraditional solid particles because of their associated low densities.Hollow spheres may also be used in applications as diverse ashierarchical filtration membranes and proppants to prop open fracturesin subterranean formations.

Ceramic proppants are widely used as propping agents to maintainpermeability in oil and gas formations. Conventional proppants offeredfor sale exhibit exceptional crush strength but also extreme density.Typical densities of ceramic proppants exceed 100 pounds per cubic foot.Proppants are materials pumped into oil or gas wells at extreme pressurein a carrier solution (typically brine) during the fracturing process.Once the pumping-induced pressure is removed, proppants “prop ” openfractures in the rock formation and thus preclude the fracture fromclosing. As a result, the amount of formation surface area exposed tothe well bore is increased, enhancing recovery rates. Proppants also addmechanical strength to the formation and thus help maintain flow ratesover time. Three grades of proppants are typically employed: sand,resin-coated sand and ceramic proppants. Proppants are principally usedin gas wells, but do find application in oil wells.

Relevant quality parameters include: particle density (low density isdesirable), crush strength and hardness, particle size (value depends onformation type), particle size distribution (tight distributions aredesirable), particle shape (spherical shape is desired), pore sizedistribution (tight distributions are desirable), surface smoothness,corrosion resistance, temperature stability, and hydrophilicity(hydro-neutral to phobic is desired).

Ceramic proppants dominate sand and resin-coated sand on the criticaldimensions of crush strength and hardness. They offer some benefit interms of maximum achievable particle size, corrosion and temperaturecapability. Extensive theoretical modeling and practical case experiencesuggest that conventional ceramic proppants offer compelling benefitsrelative to sand or resin-coated sand for most formations.Ceramic-driven flow rate and recovery improvements of 20% or morerelative to conventional sand solutions are not uncommon.

Ceramic proppants were initially developed for use in deep wells (e.g.,those deeper than 7,500 feet) where sand's crush strength is inadequate.In an attempt to expand their addressable market, ceramic proppantmanufacturers have introduced products focused on wells of intermediatedepth.

Resin-coated sands offer a number of advantages relative to conventionalsands. First, resin coated sands exhibit higher crush strength thanuncoated sand given that resin-coating disperses load stresses over awider area. Second, resin-coated sands are “tacky ” and thus exhibitreduced “proppant flow-back ” relative to conventional sand proppants(e.g. the proppant stays in the formation better). Third, resin coatingstypically increase sphericity and roundness thereby reducing flowresistance through the proppant pack.

Ceramics are typically employed in wells of intermediate to deep depth.Shallow wells typically employ sand or no proppant. As will be describedin later sections, shallow “water fracs'” represent a potential marketroughly equivalent to the current ceramic market in terms of ceramicmarket size.

With a combined annual production of over 30 million tons, the oxidesand hydroxides of aluminum are undoubtedly among the most industriallyimportant chemicals (K. Wefers and C. Misra, “Oxides and Hydroxides ofAluminum.” Alcoa Laboratories, 1987). Their uses include: precursors forthe production of aluminum metal, catalysts and absorbents; structuralceramic materials; reinforcing agents for plastics and rubbers, antacidsand binders for the pharmaceutical industry; and as low dielectric lossinsulators in the electronics industry. With such a diverse range ofapplications, it is unsurprising that much research has been focused ondeveloping and understanding methods for the preparation of thesematerials.

Traditional ceramic processing involves three basic steps generallyreferred to as powder processing, shape forming, and densification,often with a final mechanical finishing step. Although several steps maybe energy intensive, the most direct environmental impact arises fromthe shape-forming process where various binders, solvents, and otherpotentially toxic agents are added to form and stabilize a solid(“green”) body. In addition to any innate health risk associated withthe chemical processing, these agents are subsequently removed ingaseous form by direct evaporation or pyrolysis. In many cast-parts, theliquid solvent alone consists of over 50% of the initial volume ofmaterial. The component chemicals listed, with relative per percentage,in Table 1 are essentially mixed to a slurry, cast, then dried andfired. All solvents and additives must be removed as gaseous productsvia evaporation or pyrolysis.

TABLE 1 Composition of a non aqueous tape-casting alumina slurryFunction Composition Volume % Powder Alumina 27 Solvent1,1,1-Trichloroethylene/Ethyl Alcohol 58 Deflocculent Menhaden Oil 1.8Binder Polyvinyl Butyrol 4.4 Plasticizer Polyethylene Glycol/OctylPhthalate 8.8

Whereas the traditional sintering process is used primarily for themanufacture of dense parts, the solution-gelation process has beenapplied industrially primarily for the production of porous materialsand coatings. Solution-gelation involves a four-stage process:dispersion; gelation; drying; firing. A stable liquid dispersion or solof the colloidal ceramic precursor is initially formed in a solvent withappropriate additives. By change in concentration (aging) or pH, thedispersion is polymerized to form a solid dispersion or gel. The excessliquid is removed from this gel by drying and the final ceramic isformed by firing the gel at higher temperatures.

The common solution-gelation route to aluminum oxides employs aluminumhydroxide or hydroxide-based material as the solid colloid, the secondphase being water and/or an organic solvent. Aluminum hydroxide gelshave traditionally been prepared by the neutralization of a concentratedaluminum salt solution; however, the strong interactions of the freshlyprecipitated alumina gels with ions from the precursors solutions makesit difficult to prepare these gels in pure form. To avoid thiscomplication alumina gels may be prepared from the hydrolysis ofaluminum alkoxides, Al(OR)₃ (Eq. 1).

Although this method was originally reported by Adkins in 1922 (A.Adkins, J. Am. Chem. Soc. 1922, 44, 2175), it was not until the 1970'swhen it was shown that transparent ceramic bodies can be obtained by thepyrolysis of suitable alumina gels, that interest increasedsignificantly (B. E. Yoldas, J. Mat. Sci. 1975, 10, 1856).

The exact composition of the gel in commercial systems is ordinarilyproprietary, however, a typical composition can include an aluminumcompound, a mineral acid and a complexing agent to inhibit prematureprecipitation of the gel, e.g., Table 2. The aluminum compound wastraditionally assumed to be the direct precursor to pseudo-boehmite.However, the gel is now known to consist of aluminum-oxygenmacromolecular species with a boehmite-like core: alumoxanes.

TABLE 2 Typical composition of an alumina sol-gel for slipcast filtermembranes Function Composition Boehmite Precursor ASB [aluminumsec-butoxide, Al(OC₄H₉)₃] Electrolyte HNO₃ 0.07 mole/mole ASB Complexingagent glycerol ca. 10 wt. %

The replacement of 1,1,1 -trichloroethylene (TCE) as a solvent in thetraditional ceramic process must be regarded as a high priority forlimiting environmental pollution. Due to its wide spread use as asolvent in industrial processes, TCE has become one of the most commonlyfound contaminants in ground waters and surface waters. Concentrationsrange from parts per billion to hundreds of milligrams per liter. TheUnited States Environmental Protection Agency (USEPA) included TCE onits 1991 list of 17 high-priority toxic chemicals targeted for sourcereduction. The 1988 releases of TCE reported under the voluntary rightto know provisions of Superfund Amendments and Reauthorization Act(SARA) totaled to 190.5 million pounds.

The plasticizers, binders, and alcohols used in the process present anumber of potential environmental impacts associated with the release ofcombustion products during firing of the ceramics, and the need torecycle or discharge alcohols which, in the case of discharge towaterways, may exert high biological oxygen demands in the receivingcommunities.

Ceramic ultrafiltration (UF) and nanofiltration (NF) membranes have beenfabricated by the sol-gel process in which a thin membrane film isdeposited, typically by a slip-cast procedure, on an underlying poroussupport. This is typically achieved by hydrolysis of Al, Ti, Zr or othermetal compounds to form a gelatinous hydroxide at a slightly elevatedtemperature and high pH. In the case of alumina membranes, this firststep may be carried out with 2-butanol or iso-propanol. After removingthe alcohol, the precipitated material is acidified, typically usingnitric acid, to produce a colloidal suspension. By controlling theextent of aggregation in the colloidal sol, membranes of variablepermeability may be produced. The aggregation of colloidal particles inthe sol is controlled by adjusting the solution chemistry to influencethe diffuse layer interactions between particles or throughultrasonification. Alternatively, a sol gel can be employed, which isthen applied to a porous support. While this procedure offers greatercontrol over membrane pore size than does the metal precipitation route,it is nonetheless a difficult process to manipulate. In both cases,plasticizers and binders are added to improve the properties of the slipcast solution. Once the film has been applied it is dried to preventcracking and then sintered at high temperature.

The principal environmental results arising from the sol-gel process arethose associated with use of strong acids, plasticizers, binders, andsolvents. Depending on the firing conditions, variable amounts oforganic materials such as binders and plasticizers may be released ascombustion products. NOx's may also be produced from residual nitricacid in the off-gas. Moreover, acids and solvents must be recycled ordisposed of. Energy consumption in the process entails “upstream ”environmental emissions associated with the production of that energy.

The aluminum-based sol-gels formed during the hydrolysis of aluminumcompounds belong to a general class of compounds: alumoxanes. Alumoxaneswere first reported in 1958 and have since been prepared with a widevariety of substituents on aluminum. The structure of alumoxanes wasproposed to consist of linear (I) or cyclic (II) chains (S.Pasynkiewicz, Polyhedron, 1990, 9, 429). Recent work has redefined thestructural view of alumoxanes, and shown that they are not chains butthree dimensional cage compounds (A. W. Apblett, A. C. Warren, and A. R.Barron, Chem. Mater., 1992, 4, 167; C. C. Landry, J. A. Davis, A. W.Apblett, and A. R. Barron, J. Mater. Chem., 1993, 3, 597). For example,siloxy-alumoxanes, [Al(O)(OH)x(OSiR3)1 −x]n, consist of analuminum-oxygen core structure (III) analogous to that found in themineral boehmite, [Al(O)(OH)]n, with a siloxide substituted periphery.

Precursor sol-gels are traditionally prepared via the hydrolysis ofaluminum compounds (Eq. 1). This “bottom-up ” approach of reacting smallinorganic molecules to form oligomeric and polymeric materials has metwith varied success, due to the difficulties in controlling the reactionconditions, and therefore the stoichiometries, solubility, andprocessability, of the resulting gel. It would thus be desirable toprepare alumoxanes in a one-pot bench-top synthesis from readilyavailable, and commercially viable, starting materials, which wouldprovide control over the products.

In the siloxy-alumoxanes, the “organic ” unit itself contains aluminum,i.e., IV. Thus, in order to prepare the siloxy-alumoxane similar tothose previously reported the anionic moiety, the “ligand ”[Al(OH)₂(OSiR₃)₂]³¹, would be used as a bridging group; adding this unitwould clearly present a significant synthetic challenge. However, thecarboxylate-alumoxanes represent a more realistic synthetic target sincethe carboxylate anion, [RCO₂]³¹, is an isoelectronic and structuralanalog of the organic periphery found in siloxy-alumoxanes (IV and V).Based upon this rational, a “top-down ” approach has been developedbased upon the reaction of boehmite, [Al(O)(OH)]_(n), with carboxylicacids, Eq. 2 (Landry, C. C.; Pappè, N.; Mason, M. R.; Apblett, A. W.;Tyler, A. N.; MacInnes, A. N.; Barron, A. R., J. Mater. Chem. 1995, 5,331).

The carboxylate-alumoxane materials prepared from the reaction ofboehmite and carboxylic acids are air and water stable materials and arevery processable. The soluble carboxylate-alumoxanes can be dip-coated,spin coated, and spray-coated onto various substrates. The physicalproperties of the alumoxanes are highly dependent on the identity of thealkyl substituents, R, and range from insoluble crystalline powders topowders that readily form solutions or gels in hydrocarbon solventsand/or water. The alumoxanes are indefinitely stable under ambientconditions, and are adaptable to a wide range of processing techniques.Given the advantages observed for the application of carboxylatealumoxanes, e.g., the low price of boehmite ($1 kg³¹ ¹) and theavailability of an almost infinite range of carboxylic acids make thesespecies ideal as precursors for ternary and doped aluminum oxides. Thealumoxanes can be easily converted to γ-Al₂O₃ upon mild thermolysis (A.W. Apblett, C. C. Landry, M. R. Mason, and A. R. Barron, Mat. Res. Soc.,Symp. Proc., 1992, 249, 75).

SUMMARY OF THE INVENTION

A feature of the present invention is to provide a proppant havingsuitable crush strength and/or buoyancy as shown by specific gravity.

A further invention of the present invention is to provide a proppantthat can overcome one or more of the disadvantages described above.

The present invention relates to a proppant comprising a templatematerial and a shell on the template material, wherein the shellcomprises a ceramic material or oxide thereof or a metal oxide. Thetemplate material can be a hollow sphere and can be a single particle,such as a cenosphere.

The present invention further relates to a proppant having a surfacethat comprises a ceramic material or oxide thereof or a metal oxide,wherein the surface has an average grain size of 1 micron or less. Otheraverage grain sizes are possible. The surface can have a maximum grainsize, as well as a tight distribution with respect to the grain sizes.

The present invention further relates to a method to prop opensubterranean formation fractions using one or more proppants, which arepreferably contained in proppant formulations.

The present invention further relates to methods of making the variousproppants of the present invention. For instance, one method includescoating a template material with a formulation comprising a ceramicmaterial or oxide thereof or a metal oxide to form a shell around thetemplate and then hardening the shell, such as by sintering orcalcining. Other methods are further described.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are intended to provide a further explanation of the presentinvention, as claimed.

The accompanying figures, which are incorporated in and constitute apart of this application, illustrate various embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic showing an embodiment of a proppant of the presentinvention showing a substrate (A) with the coating (B). The substrate(A) may be chosen from a group including, but not limited to, ceramic,natural material, shell, nut, or other materials. The coating (B) can bechosen from a group including, but not limited to, ceramic, ceramicprecursor, polymer, resin, or a nanoparticle reinforced polymer or ananoparticle reinforced resin.

FIG. 2 shows a schematic of a proppant of the present invention showinga hollow substrate (C) with the coating (B). The substrate (C) may bechosen from a group including, but not limited to, ceramic, naturalmaterial, shell, nut, or other described in the claims. The coating (B)can be chosen from a group including, but not limited to, ceramic,ceramic precursor, polymer, resin, or a nanoparticle reinforced polymeror a nanoparticle reinforced resin.

FIG. 3 shows a schematic of the reaction or conversion of the coating(B) and substrate (A) to form a mixed phase or new phase material (D).

FIG. 4 is an SEM image of pre-expanded polystyrene with 1 coat of a 10%acetate-alumoxane nanoparticle solution and heated to 220° C. for 1hour.

FIG. 5 is a SEM image of cenosphere with a coating of 10% acetatealumoxane nanoparticle solution heated to 1000° C. for 1 hour.

FIGS. 6-11 are SEM microphotographs of several embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The materials of the present invention when used as proppants coulddominate current proppant solutions on all relevant quality dimensions.The methods of this invention are aimed at the fabrication of proppantsthat preferably exhibit neutral buoyancy, high crush strength, highsphericity, narrow size distribution, and/or high smoothness. Thesematerials have the ability to materially reduce and/or possiblyeliminate the need to employ expensive and reservoirpermeability-destroying polymer carrier gels.

Equally important, the optimal shape, size, size distribution, pore sizedistribution, and/or surface smoothness properties of the presentinvention suggest that flow resistance through the proppant pack couldbe reduced, such as by more than 50%. Neutral buoyancy enhances proppanttransport deep into the formation increasing the amount of fracture-areapropped thereby increasing the mechanical strength of the reservoir. Dueto the above issues, proppants of the present invention can achievesubstantially increased flow rates and/or enhanced hydrocarbon recovery.The low-cost of the present invention's preferred nanoparticles, and thereduced material requirements (on a per pound basis) are advantages ofthe present invention's preferred proppants. The low density of thepresent invention's proppants may enable reductions in transportationcosts in certain situations.

The proppants of the present invention present oil and gas producerswith one or more of the following benefits: improved flow rates,improved productive life of wells, improved ability to design hydraulicfractures, and/or reduced environmental impact. The proppants of thepresent invention are designed to improve flow rates, eliminating ormaterially reducing the use of permeability destroying polymer gels,and/or reducing pressure drop through the proppant pack, and/or theability to reduce the amount of water trapped between proppants therebyincreasing hydrocarbon “flow area.”

The high density of conventional ceramic proppants and sands (roughly100 lb/cu.ft.) inhibit their transport inside fractures. High densitycauses proppants to “settle out ” when pumped thereby minimizing theirefficacy. To maintain dense proppants in solution, expensive polymergels are typically mixed with the carrier solution (e.g. completionfluid). Once suspended in a gelled completion fluid, proppant transportis considerably enhanced.

Polymer gels are extremely difficult to de-cross link, however. As aresult, the gel becomes trapped downhole, coats the fracture, andthereby reduces reservoir permeability. Gel-related reservoirpermeability “damage factors ” can range from 40% to more than 80%depending on formation type.

The neutral buoyancy property that can be exhibited by the proppants ofthe present invention preferably eliminates or greatly reduces the needto employ permeability destroying polymer gels as they naturally stay insuspension.

Equally important, the shape and surface properties of the proppants ofthe present invention preferably reduce the pressure drop through theproppant pack. As a result, flow rates should increase. Theoreticalmodeling of the non-linear non-darcy flow effects (reduced beta factor)associated with the proppants of the present invention show that thisbenefit could be significant—perhaps more than a 50% reduction inproppant pack flow resistance. Key details include improved sphericityand roundness, improved surface smoothness, and/or near-monodispersesize distribution.

In one or more embodiments, the proppants of the present invention aredesigned to improve reservoir flow rates by changing the hydrophilicproperties of the proppants themselves. The hydrophilic nature ofcurrent proppants causes water to be trapped in the pore spaces betweenproppants. If this water could be removed, flow rates would beincreased.

The use of extreme pressure, polymer gels, and/or exotic completionfluids to place ceramic proppants into formations adversely impacts themechanical strength of the reservoir and shortens its economic life.Proppants of the present invention preferably enable the use of simplercompletion fluids and possibly less (or slower) destructive pumping.Thus, reservoirs packed with neutrally buoyant proppants preferablyexhibit improved mechanical strength/permeability and thus increasedeconomic life.

More importantly, enhanced proppant transport enabled by neutralbuoyancy preferably enable the placement of the proppant of the presentinvention in areas that were heretofore impossible, or at least verydifficult to prop. As a result, the mechanical strength of the formationis preferably improved, preferably reducing decline rates over time.This benefit could be of significant importance—especially within “waterfracs ” where the ability to place proppants is extremely limited.

If neutrally buoyant proppants are employed, water (fresh to heavybrines) may be used in place of more exotic completion fluids. The useof simpler completion fluids would reduce or eliminate the need toemploy de-crossing linking agents. Further, increased use ofenvironmentally friendly proppants may reduce the need to employ otherenvironmentally damaging completion techniques such as flashingformations with hydrochloric acid.

In addition to fresh water, salt water and brines, or synthetic fluidsare sometimes used in placing proppants to the desired locations. Theseare of particular importance for deep wells.

In the present invention a range of approaches for the synthesis andfabrication of proppants with designed buoyancy are disclosed. Theproppants are designed such that the material properties are such thatthe proppant preferably has neutral, positive, or negative buoyancy inthe medium chosen for pumping the proppant to its desired location inthe subterranean formation.

In the present invention, the proppant can be either solid throughout orhollow within the proppant to control buoyancy. In the presentinvention, a solid proppant is defined as an object that does notcontain a void space in the center, although a porous material would besuitable. A fully dense material is not a requirement of solid. A hollowmaterial is defined as a object that has at least one void space insidewith a defined size and shape.

In the present invention the proppant can be made from a ceramic, apolymer, or mixture thereof. The proppant can be made fromnanoparticles. The proppant can be a composite or combination ofceramic, polymer and other materials. Although not required it isunderstood that a ceramic may include oxides such as aluminum oxides(alumina) or mixed metal aluminum oxides (aluminates).

The strength properties for a proppant can be dependent on theapplication. It is intended that a crush strength of 4000 psi to 12,000psi or higher is desirable. However, for specific applications,crush-strengths of greater than 9000 psi or greater than 12000 psi aredesirable. Other crush strengths below or above these ranges arepossible.

The optimum size of the proppant can also be dependent on the particularapplication. Part of the present invention is that it is possible todesign various proppant sizes. Sizes (e.g., particle diameters) may varyfrom 10 μm to 10,000 μm. The particle diameter can be in the range offrom 50 μm to 2,000 μm.

Although the proppant can be made from a single-phase material or can bemade from a multi-phase system, such as from a two phase system thatcomprises a substrate (or template) and a second phase. A summary ofexemplary templates and substrates is shown in Table 3.

The substrate or template may be an inorganic material such as a ceramicor glass. Specifically, the ceramic can be an oxide such as aluminumoxides (alumina) as well as mixed metal aluminum oxides such as metalaluminates containing calcium, yttrium, titanium, lanthanum, barium,and/or silicon in addition to aluminum. In order to make variablebuoyant proppants, it is preferable to use a ceramic cenosphere orsimilar glass-like hollow sphere as the substrate or template.

Alternatively, the substrate may be a polymer or organic molecule orsurfactant. Although not limited as such, the polymer may be chosen frompolystyrene, latex, polybutadiene, polyethylene, polypropylene andchemically related polymers. The polymer can be a naturally occurringmaterial such as a peptide or protein.

Alternatively, the substrate can be a naturally occurring materialchosen from plant or tree material, such as plant seeds, crushed nuts,whole nuts, plant pips, cells, coffee grinds, or food products.

In a two-phase system, the second phase can coat the supporting ortemplate first phase, or infiltrates the supporting or template firstphase, or reacts with the supporting or template first phase.

The second phase can be a polymer, such as an epoxide, polyolefin, orpolymethacrylate. Furthermore, a nanopartcle material such as analumoxane optionally containing chemical functional groups that allowfor reaction with the polymer can reinforce the polymer. Suitablechemical functional groups or substituents include, but are not limitedto, hydroxides, amines, carboxylates, or olefins.

The second phase can also be a ceramic or glass. The ceramic can be anoxide, such as aluminum oxide called alumina, or a mixed metal oxide ofaluminum called an aluminate, a silicate, or an aluminosilicate, such asmullite or cordierite. The aluminate or ceramic may contain magnesium,calcium, yttrium, titanium, lanthanum, barium, and/or silicon. Theceramic may be formed from a nanoparticle precursor such as analumoxane. Alumoxanes can be chemically functionalized aluminum oxidenanoparticles with surface groups including those derived fromcarboxylic acids such as acetate, methoxyacetate, methoxyethoxyacetate,methoxyethoxyethoxyacetate, lysine, and stearate, and the like.

The designed proppant can be suspended in a liquid phase. The liquidphase may make the proppant more easy to transport to a drill site.Transportation may be by rail transport, road or ship, or any otherappropriate method, depending on geography and economic conditions. Inaddition to transport to the drill site, the suspended mixture ispreferably pumpable or otherwise transportable down the well to asubterranean formation and placed such as to allow the flow ofhydrocarbons out of the formation.

Specific methods for designing proppants with specific buoyancy,strength, size, and/or other desirable properties are summarized below.

A proppant particle with controlled buoyancy and crush strength used toprop open subterranean formation fractures can be made from a naturallyoccurring substrate coated with an organic polymer or resin coatingpreferably containing a nanoparticle reinforcement. The naturallyoccurring substrate can be chosen from the following group: crushed nutshells, plant seeds, coffee grinds, plant pips, or other food products.The organic polymer or resin can be chosen from the following group:epoxide resin, polyethylene, polystyrene, or polyaramide. Thenanoparticle reinforcement can be of various types, but is preferably acarboxylate alumoxane in which the carboxylate alumoxane optionally hasone or more types of chemical functional groups that can react orotherwise interact with the polymer resin and/or also allow for thealumoxane to be miscible with the polymer. Proppants of this design maybe made by suspending a substrate material in a suitable solvent, addingthe polymer, resin or resin components, adding the nanoparticle,allowing the resin and nanoparticle mixture to coat the substratematerial, and drying the coated particle. The nanoparticle and resincomponents can be pre-mixed before addition to the substrate, and asolvent or other components can be a component of the resin or polymer.

A proppant particle with controlled buoyancy and crush strength used toprop open subterranean formation fractures can be made from a ceramicsubstrate and an organic polymer or resin coating. The ceramic substrateor template can be a non-porous or porous particle and can be a solid orhollow particle. It is preferable that the particle is a hollowspherical particle such as a cenosphere or similar product. Cenospherescan be commercially produced ceramic or glass hollow spheres that aremade as side products in various industrial processes. The organicpolymer or resin can be chosen from the following group: epoxide resin,polyethylene, polystyrene, or polyaramide. Proppants of this design maybe made by suspending a substrate material in a suitable solvent, addingthe polymer, resin or resin components, allowing the resin to coat thesubstrate material, and drying the coated particle. It is possible touse a solvent to facilitate the coating process. An improved version ofthis proppant can be prepared by the addition of nanoparticles forreinforcement, such as an alumoxane optionally with chemical functionalgroups that react and/or allow miscibility with the polymer resin. Analternative method of controlling the properties of the proppant is toadd a linker group to the surface of the ceramic substrate that canreact with the organic polymer or resin coating.

A proppant particle with controlled buoyancy and/or crush strength usedto prop open subterranean formation fractures can be made from a ceramicsubstrate, a ceramic coating, or infiltration. The ceramic substrate ortemplate is preferably spherical and hollow such as a cenosphere orsimilar material. However, any suitable substrate that provides theresulting properties of the proppant may be used. The ceramic coating orinfiltration can be an oxide, for instance, an oxide of aluminum or amixed metal oxide of aluminum. A proppant of this type may be preparedby coating of a spherical template with a ceramic precursor solution,drying the coated ceramic particle, and heating the coated ceramicparticle to a temperature sufficient to form ceramic sphere of desiredporosity and hardness. The ceramic precursor may be a nanoparticle suchas an alumoxane, or a sol-gel precursor. Proppants of this type may beprepared by suspending the ceramic substrate in a suitable solvent,adding a ceramic precursor, allowing the ceramic precursor to coat theceramic substrate, drying the coated ceramic particle, and heating thecoated ceramic particle to a temperature sufficient to form ceramicspheres of desired porosity and hardness.

TABLE 3 Possible Templates for proppants Food and food Plants andproducts minerals Waste Other Coffee, Soils, Slag (steel, coke) a)Polypropylene, Milk, Bauxite, Silicas b) Glass Beads Whey, Cellulose,(diatomaceous c) Surfactants/ Animal/Fish Guargum, earth, diatomite,Detergents Eggs, Algae, kesselgur, d) Polystyrene Nuts, Lignin, PoppedPerlite, e) Bacteria Small corn Poppy Seeds, Vermiculate), f) Eraserspieces. Mustard Seeds Fly ash (coke), g) Soap Sawdust Wood flour, RapeSeeds Rust, h) macrolite Grain Husks Plankton Gypsum (fertilizer),(corn, maize, Pieces of Rubber (tires), mailo, sponge Spent FCCcatalyst, capher, Fur/Hair, Spent Motor Oil sorgum). Kohl Usedadsorbents Rabi Seeds Flue gas filter cakes from bags in baghousescenospheres

In the present invention, in one or more embodiments, the inventionrelates to a proppant used to prop open subterranean formation fractionscomprising a particle or particles with controlled buoyancy and/or crushstrength. The controlled buoyancy can be a negative buoyancy, a neutralbuoyancy, or a positive buoyancy in the medium chosen for pumping theproppant to its desired location in the subterranean formation. Themedium chosen for pumping the proppant can be any desired medium capableof transporting the proppant to its desired location including, but notlimited to a gas and/or liquid, like aqueous solutions, such as water,brine solutions, and/or synthetic solutions. Any of the proppants of thepresent invention can have a crush strength sufficient for serving as aproppant to prop open subterranean formation fractures. For instance,the crush strength can be 3,000 psi or greater, greater than 4000 psi,greater than 9000 psi, or greater than 12000 psi. Suitable crushstrength ranges can be from about 3000 psi to about 15000 psi, or fromabout 5000 psi to about 15000 psi, and the like.

The proppants of the present invention can comprise a single particle ormultiple particles and can be a solid, partially hollow, or completelyhollow in the interior of the particle. The particle can be spherical,nearly spherical, oblong in shape (or any combination thereof) or haveother shapes suitable for purposes of being a proppant.

The proppant can have any particle size. For instance, the proppant canhave a particle diameter size of from about 1 nm to 1 cm or a diameterin the range of from about 1 micron to about 1 mm, or a diameter of fromabout 10 microns to about 10000 microns, or a diameter of from about1000 microns to about 2000 microns. Other particle sizes can be used.Further, the particle sizes as measured by their diameter can be abovethe numerical ranges provided herein or below the numerical rangesprovided herein.

In one or more embodiments of the present invention, the particlecomprising the proppant can be or can contain a ceramic material. Theceramic material can comprise an oxide such as an oxide of aluminum. Theceramic material can comprise an aluminate. For instance, the aluminatecan be an aluminate of calcium, yttrium, titanium, lanthanum, barium,silicon, any combinations thereof, and other elements that can formaluminates.

In the present invention, the particle(s) forming the proppant cancomprise a substrate or template and a second phase, such as a coatingon the substrate or template. The substrate or template can be a polymeror surfactant or ceramic material. The polymer, for instance, can be anythermoplastic or thermoset polymer, or naturally occurring polymer. Forinstance, the polymer can be a polystyrene, a latex, or polyalkylene,such as a polyethylene or polypropylene. The polymer can be apolybutadiene, or related polymers or derivatives of any of thesepolymers. The polymer can be a naturally occurring material or cancontain a naturally occurring material. For instance, the naturallyoccurring material can be a peptide or protein, or both.

With respect to the substrate or template, the substrate or template canbe a naturally occurring material or can contain a naturally occurringmaterial. For instance, the naturally occurring material can be a plantmaterial or tree material. For instance, the naturally occurringmaterial can be a plant seed, a crushed nut, whole nut, plant pip, cell,coffee grind, or food products, or any combination thereof. The ceramicmaterial can comprise a cenosphere.

The second phase or coating, or shell can coat the template orsubstrate. The second phase or template can infiltrate the template orsubstrate. Further, or in the alternative, the second phase or shell orcoating can react with the substrate or template, or a portion thereof.

The second phase, coating, or shell, can comprise one or more polymerssuch as a thermoplastic or thermoset polymer(s). Examples include, butare not limited to, an oxide, polyolefin, polymethacrylate, and thelike. The coating, shell, or second phase can optionally be reinforcedby nanoparticles. The nanoparticle material can be any type of materialcapable of acting as a reinforcement material. Examples include, but arenot limited to, ceramics, oxides, and the like. Specific examplesinclude, but are not limited to, alumoxane. The alumoxane can optionallycontain one or more chemical functional groups that are on thealumoxane. These chemical functional groups can permit, facilitate, orotherwise permit reaction with a polymer that also forms the coating orshell, or the polymer that may be present in the template or substrate.Examples of substituents that may be on the nanoparticles, such as thealumoxane, include, but are not limited to, hydroxides, amines,carboxylates, olefins, and/or other reactive groups, such as alkylgroups, aromatic groups, and the like.

The coating or shell or second phase can be or contain a ceramicmaterial(s), such as an oxide(s). Specific examples include, but are notlimited to, an oxide(s) of aluminum. The ceramic can be an aluminate oralumina. For instance, the aluminate can be an aluminate of calcium,yttrium, titanium, lanthanum, barium, silicon, or any combinationthereof, or can contain other elements. The material forming the coatingor shell can be initially in the form of a nanoparticle such as analumoxane. The alumoxane can be acetate, methoxyacetate,methoxyethoxyacetate, methoxyethoxyethoxyacetate, lysine, stearate, orany combination thereof.

In one or more embodiments of the present invention, the proppant can besuspended in a suitable liquid phase. The liquid phase is generally onethat permits transport to a location for use, such as a well site orsubterranean formation. For instance, the subterranean formation can beone where proppants are used to improve or contribute to the flow ofhydrocarbons, natural gas, or other raw materials out of thesubterranean formation. In another embodiment of the present invention,the present invention relates to a well site or subterranean formationcontaining one or more proppants of the present invention.

In one embodiment of the present invention, the proppant whichpreferably has controlled buoyancy and/or crush strength has a naturallyoccurring substrate or template with an organic polymer or resin coatingon the template or substrate and wherein the organic polymer or resincoating contains nanoparticles, preferably for reinforcement purposes.As specific examples, but non-limiting examples, the naturally occurringsubstrate can be a crushed nut, cell, plant seed, coffee grind, planttip, or food product. The organic polymer or resin coating, forinstance, can be an epoxy resin, polyethylene, polystyrene, orpolyaramide, or other thermoplastic or thermoset polymers. Thenanoparticle can be an alumoxane, such as an carboxylate alumoxane orother ceramic material. The alumoxane can have one or more chemicalfunctional groups that are capable of reacting with the organic polymeror resin coating. The functional groups can optionally allow the ceramicmaterials, such as alumoxane, to be miscible with the polymer or resincoating. The crush strength of this proppant can be as describedearlier. The proppant can have a diameter as described earlier or can bea diameter in the size range of from about 25 to about 2000 microns.Other diameter size ranges are possible.

In the present invention, the template or substrate can be a ceramicmaterial with an organic polymer or resin coating. The ceramic substratecan be a porous particle, substantially non-porous particle, or anon-porous particle. The ceramic template or substrate can be spherical.The ceramic substrate can be a hollow particle. For instance, theceramic substrate can be a cenosphere. The organic polymer or resincoating can be as described above. The crush strength can be the same asdescribed above. The diameter can be the same as described earlier.Optionally, the proppant can have nanoparticles for reinforcement valueor other reasons. The nanoparticle can be in the polymer or resincoating. The nanoparticle can be the same as described earlier.

In another embodiment, the proppant can have a substrate or templatecontaining or made from one or more ceramic material(s). A linker groupcan be located on the template or substrate. A shell or coatingcontaining a polymer containing a resin coating can be located aroundthis template or substrate having the linker group. More than one typeof linker group can be used. The linker group, in at least oneembodiment, permits bonding between the substrate or template and thecoating. The linker group can be a coupling agent. The coupling agentcan be of the type used with metal oxides.

In another embodiment, the proppant can have a substrate or templatethat comprises a ceramic material and further has a coating or shellthat comprises a ceramic material that can be the same or different fromthe template material. The template or substrate and the shell orcoating can have the same characteristics and parameters as describedabove for the other embodiments, such as shape, crush strength,buoyancy, and the like. Preferably, the ceramic substrate or template isa cenosphere and the ceramic coating or shell is an oxide, such as anoxide of aluminum or aluminate, a silicate, or an aluminosilicate. Otherexamples include, but are not limited to, shells that contain silicon,yttrium, magnesium, titanium, calcium, or any combinations thereof.

The proppants of the present invention can be made a number of ways. Forinstance, the substrate material can be suspended in a suitable solventand then the material forming the shell or coating can be added to thesolvent containing the suspended substrate material. Optionally,nanoparticles can be added. The coating material, such as the polymer orresin, as well as the nanoparticle(s) present as a mixture can then coatthe substrate material. Afterwards, the coated particle is dried usingconventional drying techniques such as an oven or the like. The optionalpresence of nanoparticles can optional react with the coating material,such as the polymer or resin. Furthermore, if nanoparticles are used,the nanoparticles can be added separately or can be pre-mixed with thecoating components, such as the resin or polymer, prior to beingintroduced to the suspension of substrate material. The solvent that isused to suspend the substrate material can be part of or present withthe polymer or resin coating materials. Furthermore, the coatingmaterials can optionally cross link during the coating process toperform a cross-linked coating on the substrate or template.

As another option, if a linkage molecule or material is used, thelinkage molecule can be reacted with the substrate or template prior tobeing suspended in a solvent, or after the substrate material issuspended in a solvent. The linkage molecule optionally reacts with thesubstrate or template material and optionally reacts with the coating orshell material such as the resin or polymer. Again, nanoparticles can beadded at any point of this process.

In another method of making one or more types of proppants of thepresent invention, a template or substrate material can be coated, suchas with a precursor solution such as a ceramic containing precursorsolution. The coated template material can then be dried andsubsequently heated to a temperature to form a densified shell, forinstance, having desirable porosity or hardness, or both. Preferably,the material is in the shape of a sphere. In this embodiment, theprecursor solution preferably comprises nanoparticles such as ceramicnanoparticles. For instance, ceramic particles can be alumoxane. Theprecursor solution can be in the form of a sol-gel. For instance, thesol-gel can contain aluminum as well as other elements. The template orsubstrate can be a hollow particle and/or can be spherical in shape. Thecoating that coats the ceramic template can optionally react with thesubstrate, for instance, during the heating step.

In another embodiment of the present invention, the proppant can beobtained by suspending a substrate, such as a ceramic substrate in asuitable solvent such as water or other aqueous solutions. The ceramicprecursor which coats the template or substrate can be added. Theceramic precursor then coats the substrate or template and then thecoated particle such as the coated ceramic particle can then be driedand subjected to heating to a temperature to form a densified materialhaving desirable porosity and/or hardness. The types of materials,characteristics, and parameters of the starting materials and finishedcoated particles as described above apply equally here in theirentirety.

In a more preferred embodiment, a solid or hollow alumina,aluminosilicate, or metal aluminate ceramic sphere is obtained bycoating a spherical template with an alumoxane solution or metal dopedalumoxane and then subsequent application of heat to convert this sphereto alumina, aluminosilicate, or a metal aluminate. The alumoxane cancomprise acetate-alumoxane. The spherical template preferably has adiameter in the size range of from about 25 to 2000 microns. The solidor hollow spherical templates can be ceramic or can be polystyrene orother polymeric materials. Even more preferably, the templates arecenospheres or synthetically produced microspheres such as thoseproduced from a blowing process or a drop tower process. In oneembodiment, the solid or hollow spherical templates remain in tactduring the conversion process to alumina, aluminosilicate, or metalaluminate. In one or more embodiments, the solid or hollow sphericaltemplates pyrolize, decompose, or are otherwise removed during theconversion process to alumina, aluminosilicate, or metal aluminate. Thewall thickness can be of any desirable thickness. For instance, the wallthickness can be in a range of from about 25 to about 2000 microns. Asan option, the surface of the formed alumina, aluminosilicate, or metalaluminate sphere can be functionalized with a chemical moiety orchemical material, such as an organic ligand, like a surfactant, and canprovide surface wetting properties which can assist in allowingadditional ceramic precursor, which is the same or different from theearlier coating, to be applied. Then, additional heat conversion canoccur to form the second or multiple coating or shell on the alreadycoated particle.

In another embodiment, the solid or spherical templates can be firstcoated with a resin or polymer and cured and then an alumoxane precursoror other similar type of precursor can be subsequently coated onto theparticle followed by heat conversion to form a sphere comprised of anouter alumina, aluminosilicate, or metal aluminate shell or similar typeof metal containing coating. This resin coating or polymer coating canpyrolize, decompose, or otherwise be removed during the conversionprocess. The coating used to coat the particles such as a solution ofalumoxane nanoparticles can contain, for instance, from about 0.5 toabout 20% alumoxane by weight of the coating solution. Other weights arepossible and permissible. The coating of the particles can occur such asby dipped coating, pan, Muller mixing, or fluid bed coating.

With respect to the polymers or resins that can be used to coat theparticles, these polymers include, but are not limited to, epoxies,polyurethanes, phenols, ureas, melamine formaldehyde, furans, syntheticrubber, natural rubber, polyester resins, and the like.

The proppants of the present invention while preferably used to propopen subterranean formation fractions, can be used in othertechnologies, such as an additive for cement or an additive forpolymers, or other materials that harden, or would benefit. Theproppants of the present invention can also be used as encapsulateddelivery systems for drugs, chemicals, and the like.

In another method of making the proppants of the present invention, acolloidal suspension containing polymeric beads can be suspended in anysolution such as an aqueous solution of a nanostructured coatingmaterial. The beads can then be covered with a nanostructured coatedmaterial to create a ceramic. The beads can then be dried andsubsequently heated to a first temperature which is sufficient toconvert the nanostructured coating material to a ceramic coating, suchas a densified coating. The temperature is preferably not sufficient todecompose the polymeric beads. Then, the polymeric beads can bedissolved, such as in a solvent, and extracted from the ceramic coating.Afterwards, the material can then be heated to a second temperature toform a hollow ceramic sphere of the desired porosity and/or strength.The nanostructure coating material can be as described above earlier,such as titania, alumina, chromium, molybdenum, yttrium, zirconium, orthe like, or any combination thereof. The nanostructure coating materialdispersed in the solution can be achieved using a sol-gel method,controlled flow cavitation, PVD-CVD, flame, plasma, high energy ballmilling, or mechanomade milling processes. The nanostructure coatingmedia can be a solution, such as alcohol, liquid hydrocarbon, orcombinations thereof.

In the present invention, the strength of the particle can be controlledby varying the wall thickness, the evenness of the wall thickness, thetype of nanoparticles used, or any combination thereof. Further, thesize of the particle can be controlled by varying the type, size, or anycombination thereof of the template used. The template can have a sizeof from about 1 nm to about 3000 microns.

In the present invention, in one or more embodiments, the templatematerial can be selected from wax, surfactant-derived liquid beads,seeds, shells, nuts, grain husks, grains, soils, powdered, ground, orcrushed agglomerates of wood products, powdered, ground, or crushedagglomerates of ceramic material, powdered, ground, crushed, or rolledorganics, silicas (glass beads), whey, cellulose, fly ash, animal eggs,rust, soap, bacteria, algae, and rubber.

More particular examples or seeds are rape seed, a poppy seed, a mustardseed, a kohl rabbi seed, a pea seed, a pepper seed, a pumpkin seed, anoil seed, a watermelon seed, an apple seed, a banana seed, an orangeseed, a tomato seed, a pear seed, a corn seed.

More particular examples of shells are walnut shell, peanut shell,pasticcio shell, or an acorn shell. More specific examples of grainhusks include, corn, maize, mailo, capher, or sorgum.

Another way to coat a particle in the present invention, can be with afluidized bed, spray drying, rolling, casting, thermolysis, and thelike.

Examples of powdered agglomerates of organic material include powderedmilk, animal waste, unprocessed polymeric resins, animal hair, plantmaterial, and the like. Examples of animal eggs include, but are notlimited to, fish, chicken, snake, lizard, bird eggs, and the like.Examples of cellulose templates include, but are not limited to algae,flower, plankton, ground cellulose such as saw dust, hay, or othergrasses, and the like. In general, the material coated can have a sizeof from about 100 to about 10,000 microns.

While the various embodiments of the present invention have beendescribed in considerable detail, the following provides additionaldetails regarding various embodiments of the present invention. It isnoted that the above disclosure of the proppants, methods of making, anduses applies equally to the following disclosure of various embodimentsof the present invention. Equally so, the following disclosure alsoapplies to the above embodiments of the present invention. Thesedisclosures are not exclusive of each other.

In one or more embodiments of the present invention, the presentinvention relates to a proppant comprising a template material and ashell on the template material. The shell can comprise a ceramicmaterial or oxide thereof or a metal oxide. The shell can contain one ormore types of ceramic material, or oxides thereof, or metal oxides, orany combinations thereof. The metal oxide can be a mixed metal oxide ora combination of metal oxides.

The template material can be porous, non-porous, or substantiallynon-porous. For purposes of the present invention, a substantiallynon-porous material is a material that is preferably at least 80%non-porous in its entirety, more preferably, at least 90% non-porous.The template material can be a hollow sphere or it can be a closed foamnetwork, and/or can be a non-composite material. A non-compositematerial, for purposes of the present invention is a material that isnot a collection of particles which are bound together by some binder orother adhesive mechanism. The template material of the present inventioncan be a single particle. In other words, the present invention canrelate to a plurality of proppants, wherein each proppant can consistsof a single particle. In one or more embodiments of the presentinvention, the template material can be a cenosphere or a syntheticmicrosphere such as one produced from a blowing process or a drop towerprocess.

Though optional, the template material can have a crush strength of 5000psi or less, 3000 psi or less, or 1000 psi or less. In the alternative,the template material can have a high crush strength such as 3000 psi ormore, such as from about 5000 psi to 10,000 psi. For purposes of thepresent invention, crush strength is determined according to APIPractice 60 (2^(nd) Ed. December 1995). In one or more embodiments ofthe present invention, the template material having a low crush strengthcan be used to provide a means for a coating to be applied in order toform a shell wherein the shell can contribute a majority, if not a highmajority, of the crush strength of the overall proppant.

The template material can optionally have voids and these voids can bestrictly on the surface of the template material or strictly in theinterior of the template material or in both locations. As describeearlier, the shell can be sintered which can form a densified materialwhich preferably has high crush strength. For instance, the shell cancomprise sintered nanoparticles. These nanoparticles can be quite small,such as on the order of 0.1 nm up to 150 nm or higher with respect toprimary particle size. The nanoparticles can comprise primary particlesalone, agglomerates alone, or a combination of primary particles andagglomerates. For instance, the primary particles can have an averageparticle size of from about 1 nm to about 150 nm and the agglomeratescan have an average particle size of from about 10 nm to about 350 nm.The weight ratio of primary particles to agglomerates can be 1:9 to 9:1or any ratio in between. Other particle size ranges above and belowthese ranges can be used for purposes of the present invention. Theshell of the proppant can have an average grain size of about 10 micronsor less. The shell of the present invention can have an average grainsize of 1 micron or less. The shell of the proppant of the presentinvention can have an average grain size of from 0.1 micron to 0.5micron. In any of these embodiments, the maximum grain size can be 1micron. It is to be understood that maximum size refers to the highestgrain size existing with respect to measured grain sizes. With respectto any of these embodiments, as an option, at least 90% of all grainsizes can be within the range of 0.1 to 0.6 micron.

With respect to the shell, the shell can further comprise additionalcomponents used to contribute one or more properties to the shell orproppant. For instance, the shell can further comprise at least onesintering aid, glassy phase formation agent, grain growth inhibitor,ceramic strengthening agent, crystallization control agent, and/or phaseformation control agent, or any combination thereof. It is to beunderstood that more than one of any one of these components can bepresent and any combination can be present. For instance, two or moresintering aids can be present, and so on. There is no limit to thecombination of various agents or the number of different agents used.Generally, one or more of these additional agents or aids can includethe presence of yttrium, zirconium, iron, magnesium, alumina, bismuth,lanthanum, silicon, calcium, cerium, one or more silicates, one or moreborates, or one or more oxides thereof, or any combination thereof.These particular aids or agents are known to those skilled in the art.For instance, a sintering aid will assist in permitting uniform andconsistent sintering of the ceramic material or oxide. A glassy phaseformation agent, such as a silicate, generally enhances sintering byforming a viscous liquid phase upon heating in the sintering process. Agrain growth inhibitor will assist in controlling the overall size ofthe grain. A ceramic strengthening agent will provide the ability tostrengthen the overall crush strength of the shell. A crystallizationcontrol agent will assist in achieving the desired crystalline phase ofthe shell upon heat treatment such as sintering or calcining. Forinstance, a crystallization control agent can assist in ensuring that adesirable phase is formed such as an alpha aluminum oxide. A phaseformation control agent is the same or similar to a crystallizationcontrol agent, but can also include assisting in achieving one or moreamorphous phases (in addition to crystalline phases), or combinationsthereof. The various aids and/or agents can be present in any amounteffective to achieve the purposes described above. For instance, the aidand/or agents can be present in an amount of from about 0.1% to about 5%by weight of the overall weight of the shell. The shell(s) can compriseone or more crystalline phases or one or more glassy phases orcombinations thereof.

The template material can be a synthetic ceramic microsphere such as oneproduced from a blowing process or a drop tower process or can be acenosphere such as a hollow cenosphere. The template material can be afly ash particle or particles and can be a particle or particles derivedfrom fly ash. In more general terms, the template material can be ahollow spherical particle. The template material can be a precipitatorfly ash. The template material can be a blown hollow sphere. In otherwords, the hollow sphere can be naturally occurring or synthetic or canbe a combination.

In one or more embodiments of the present invention, the shell can besubstantially non-porous. For instance, substantially non-porous meansthat at least 90% of the surface of the shell is non-porous.

The shell can be substantially uniform in thickness around the entireouter surface of the template material. For instance, the thickness ofthe shell can be substantially uniform in thickness by not varying inthickness by more than 20% or more preferably by not varying more than10% in overall thickness around the entire circumference of the shell.The shell can be non-continuous or continuous. Continuous, for purposesof the present invention means that the shell entirely encapsulates orcovers the template material within the shell. Preferably, the shellfully encapsulates the template material.

With respect to the shell and the interaction of the shell and thetemplate material, the shell can essentially be a physical coating onthe template material and not react with the template material.Alternatively, the shell can react with one or more portions of thetemplate material such as by chemically bonding to the templatematerial. This chemical bonding may be ionic or covalent or both. As analternative, the shell or portion thereof can diffuse, infiltrate,and/or impregnate at least a portion of the template material. Asanother alternative, the shell or at least a portion thereof can adsorbor absorb onto the template material or a portion thereof.

With respect to the outer surface of the template material and theshell, the shell can be in direct contact with the outer surface of thetemplate material. Alternatively, one or more intermediate layers can bepresent in between the outer surface of the template material and theinner surface of the shell. The intermediate layer or layers can be ofany material, such as a polymer, resin, ceramic material, oxidematerial, or the like.

The proppants of the present application can, for instance, have aspecific gravity of from about 0.6 g/cc to about 2.5 g/cc. The specificgravity can be from about 1.0 g/cc to about 1.3 g/cc or can be from abut0.9 g/cc to about 1.5 g/cc. Other specific gravities above and belowthese ranges can be obtained.

The proppant can have any of the crush strengths mentioned above, suchas 3000 psi or greater, 5000 psi to 10,000 psi, 10,000 psi to 15,000psi, as well as crush strengths above and below these ranges.

The shell of the proppant can have a wall thickness of any amount, suchas 5 microns to about 150 microns or about 15 microns to about 120microns. This wall thickness can be the combined wall thickness for twoor more shell coatings forming the shell or can be the wall thicknessfor one shell coating.

As stated, the proppant can be spherical, oblong, nearly spherical, orany other shapes. For instance, the proppant can be spherical and have aKrumbein sphericity of at least about 0.5, at least 0.6 or at least 0.7,at least 0.8, or at least 0.9, and/or a roundness of at least 0.4, atleast 0.5, at least 0.6, at least 0.7, or at least 0.9. The term“spherical ” can refer to roundness and sphericity on the Krumbein andSloss Chart by visually grading 10 to 20 randomly selected particles.The template, such as the template sphere, can have a Krumbiensphericity of at least about 0.3, or at least 0.5 or at least 0.6 or atleast 0.8 or at least 0.9, and/or a roundness of at least about 0.1, atleast about 0.3, at least about 0.5, at least about 0.7, at least about0.8, or at least about 0.9.

In one or more embodiments of the present invention, the proppant can bea spray-coated shell. The shell(s) of the proppants can be formulated byone coating or multiple coatings. More than one shell can be present inlayered constructions. The coatings can be the same or different fromeach other.

In one or more embodiments of the present invention, the shell cancomprise at least alumina, aluminosilicate, aluminate, or silicate. Forinstance, the alumina, aluminate, or silicate can contain calcium,yttrium, magnesium, titanium, lanthanum, barium, and/or silicon, or anycombination thereof. One or more rare earth metals or oxides thereof canbe present.

The template material can be from a naturally occurring material asdescribed earlier. The naturally occurring material can be seed, plantproducts, food products, and the like. Specific examples have beendescribed earlier.

The proppants of the present invention, for instance, can be made bycoating a template material with a formulation comprising a ceramicmaterial or oxide thereof or metal oxide to form a shell around thetemplate and then this formulation can be sintered to create thesintered shell having a densified structure. The shell preferably has amicrocrystalline structure. The sintering can occur at any temperatureto achieve densification of the ceramic material or oxide thereof ofmetal oxide such as from 700° C. to about 1,700° C. or about 800° C. toabout 1,700° C. Generally, sintering occurs by ramping up to thetemperature. The sintering temperature is the temperature in the oven orsintering device. As stated, the coating of the template material can beachieved by spray coating. For instance, in creating the shell, anon-alpha aluminum oxide can be coated onto a template material and thenupon sintering, form an alpha-aluminum oxide coating. The formulationcan be in the form of a slurry comprising the ceramic material or oxidethereof or metal oxide along with a carrier such as a liquid carrier.When spray coating, a spray coating chamber can be used such as a spraycoater from Vector Corporation, Model MLF.01. The formulation can beintroduced as an atomized spray and the template material is suspendedin air within the chamber during the coating of the template material.Ranges for key parameters for the spray coating process include: Airtemperature: 40°-90° C., Airflow: 90-150 liters per minute, Nozzle AirSetting: 10-25 psi. After coating, the sintering can occur.

With respect to the sintering, it is preferred that the sintering issufficient to densify the ceramic material or oxide thereof or metaloxide so as to form a continuous coating. The formulation can compriseat least one acid, surfactant, suspension aid, sintering aid, graingrowth inhibitor, glassy phase formation agent, ceramic strengtheningagent, crystallization control agent, and/or phase formation controlagent, or any combination thereof. One or more of these agents can bepresent. Again, as stated above, more than one type of the same agentcan be used such as more than one type of acid, more than one type ofsurfactant, more than one type of sintering agent, and so on. The amountof these agents can be any amount sufficient to accomplish the desiredpurposes such as from about 0.1% to about 5% by weight of the weight ofthe final shell.

As stated above, the present invention further relates to a proppantformulation comprising one or more proppants of the present inventionwith a carrier. The carrier can be a liquid or gas or both. The carriercan be water, brine, hydrocarbons, oil, crude oil, gel, foam, or anycombination thereof. The weight ratio of carrier to proppant can be from10,000:1 to 1:10,000 or any ratio in between, and preferably 0.001 lbproppant/gallon fluid to 10 lb proppant/gallon fluid.

In a more preferred example, the proppant can have the followingcharacteristics:

-   -   (a) an overall diameter of from about 90 microns to about 1,600        microns;    -   (b) spherical;    -   (c) said shell is substantially non-porous;    -   (d) said proppant has a crush strength of about 3,000 psi or        greater;    -   (e) said coating has a wall thickness of from about 15 to about        120 microns;    -   (f) said proppant has a specific gravity of from about 0.9 to        about 1.5 g/cc; and    -   (g) said template material is a hollow sphere.

Preferably, in this embodiment, the template material is a cenosphere oran aluminate, or a sintered aluminum oxide. The template materialpreferably has a crush strength of less than 3000 psi or less than 1000psi. The shell is preferably an alpha aluminum oxide coating.

For the proppants of the present invention, the shell can compriseMullite, Cordierite, or both. In one embodiment of the presentinvention, the formulation that is applied onto the template materialcan be prepared by peptizing Boehmite or other ceramic or oxidematerials with at least one acid (e.g., acetic acid) to form a sol-gelformulation comprising alumoxane. The formulation can be a slurrycomprising alumoxane along with a carrier such as a liquid carrier. Theslurry can contain one or more sintering aids, grain growth inhibitors,ceramic strengthening agents, glassy phase formation agents,crystallization control agents, and/or phase formation control agents,which can comprise yttrium, zirconium, iron, magnesium, alumina,bismuth, silicon, lanthanum, calcium, cerium, silicates, and/or borates,or oxides thereof, or any combination thereof.

As an additional embodiment, the present invention can comprise asurface that comprises a ceramic material or an oxide thereof or metaloxide wherein the surface (e.g., polycrystalline surface) has an averagegrain size of 1 micron or less. The average grain size can be about 0.5micron or less. The average grain size can be from about 0.1 micron toabout 0.5 micron. The surface having this desirable grain size can bepart of a coating, shell, or can be the core of a proppant or can be aproppant solid particle or hollow particle. The surface can have amaximum grain size of 5 microns or less, such as 1 micron. Further, thesurface can have grain sizes such that at least 90% of all grain sizesare within the range of from about 0.1 to about 0.6 micron. The proppantcan have a crush strength of 3000 psi or greater or can have any of thecrush strengths discussed above, such as 5000 psi or more or 10,000 psior more, including from about 5000 psi to about 15,000 psi. The ceramicmaterial in this proppant can further contain yttrium, zirconium, iron,magnesium, alumina, bismuth, lanthanum, silicon, calcium, cerium,silicates, and/or borates, or oxides thereof or any combination thereof.The proppants can contain one or more of sintering aid, glassy phaseformation agent, grain growth inhibitor, ceramic strengthening agent,crystallization control agent, or phase formation control agent, or anycombination thereof. The proppant formulation can contain the proppantalong with a carrier such as a liquid carrier or a gas carrier.

With respect to this embodiment, a template material is optional. Theproppant can be completely solid, partially hollow, or completelyhollow, such as a hollow sphere. If a template material is present, anyone of the template materials identified above can be used.

In all embodiments of the present invention, one or more proppants ofthe present invention can be used alone or in a formulation to prop opensubterranean formation fractions by introducing the proppant formulationinto the subterranean formation such as by pumping or other introductionmeans known to those skilled in the art. An example of a well completionoperation using a treating fluid containing proppants or particles isgravel packing. In gravel packing operations, particles referred to inthe art as gravel are carried to a subterranean producing zone in whicha gravel pack is to be placed by a hydrocarbon or water carrying fluid.That is, the particles are suspended in the carrier fluid which can beviscosified and the carrier fluid is pumped into the subterraneanproducing zone in which a gravel pack is to be placed. Once theparticles are placed in the zone, the treating fluid leaks off into thesubterranean zone and/or is returned to the surface. The gravel packproduced functions as a filter to separate formation solids fromproduced fluids while permitting the produced fluids to flow into andthrough the well bore. An example of a production stimulation treatmentutilizing a treating fluid having particles suspended therein ishydraulic fracturing. That is, a treating fluid, referred to in the artas a fracturing fluid, is pumped through a well bore into a subterraneanzone to be stimulated at a rate and pressure such that fractures areformed and extended into the subterranean zone. At least a portion ofthe fracturing fluid carries particles, referred to in the art asproppant particles into the formed fractures. The particles aredeposited in the fractures and the fracturing fluid leaks off into thesubterranean zone and/or is returned to the surface. The particlesfunction to prevent the formed fractures from closing whereby conductivechannels are formed through which produced fluids can flow to the wellbore.

While the term proppant has been used to identify the preferred use ofthe materials of the present invention, it is to be understood that thematerials of the present invention can be used in other applications,such as medical applications, filtration, polymeric applications,catalysts, rubber applications, filler applications, drug delivery,pharmaceutical applications, and the like.

U.S. Pat. Nos. 4,547,468; 6,632,527 B1; 4,493,875; 5,212,143; 4,777,154;4,637,990; 4,671,909; 5,397,759; 5,225,123; 4,743,545; 4,415,512;4,303,432; 4,303,433; 4,303,431; 4,303,730; and 4,303,736 relating tothe use of proppants, conventional components, formulations, and thelike can be used with the proppants of the present invention, and areincorporated in their entirety by reference herein. The processesdescribed in AMERICAN CERAMIC SOCIETY BULLETIN, Vol. 85, No. 1, January2006, and U.S. Pat. Nos. 6,528,446; 4,725,390; 6,197,073; 5,472,648;5,420,086; and 5,183,493, and U.S. Patent Application Publication No.2004/0012105 can be used herein and is incorporated in its entiretyherein. The proppant can be a synthetic proppant, like a syntheticcenosphere template, with any shell.

Sol-gel routes to forming monoliths or films, such as thicker than about1 micron, can suffer from severe cracking or warping during the dryingor gel consolidation process. Low solids-weight loadings result in largevolumetric shrinkage during the drying process. Furthermore, crackingcan be the result of relief of differential capillary stresses acrossthe dimensions of the gel or film as drying and shrinkage occur. Totalcapillary stress present in the film can be a function of particle size,and decreases as particle size increases. As a result, films ormonoliths formed from larger particle sizes can have a decreasedtendency to incur cracking stresses during the shrinkage and dryingprocess.

In a peptized formulation, such as a boehmite gel formulation, ablending of boehmite particles of varying dispersed size (e.g., 90%large, aggregated crystallites and 10% small, single crystallites)results in a lower number density of pores, as well as larger size ofpore in the corresponding dried gel, thereby reducing drying stress.Thus, tailoring the particle size and blend of primary particles in thesol-gel formulation can confer control over crack formation for a givendrying process. The particles have varying dispersed sizes in thesol-gel stage, but examination of the microstructure of the dried gelfragments reveals that only crystallites are distinct from one anotherin the green packing. This shows that the small particles uniformly fillthe interstices of the larger particles, resulting in a well-structuredgreen film.

In one or more embodiments, the proppants can be unagglomerated and eachproppant can have a single particulate template with a shell formed fromone or more layers. Thus, in one or more embodiments, there can be a 1:1ratio with respect to shell to template, meaning there is one shell foreach template particle or sphere. Thus, the present invention relates toa plurality of proppants which comprise individual template particles orspheres that are coated individually to form individual templates havinga shell around each template sphere or particle. Further, in one or moreembodiments, the population of proppants is consistent prior tosintering and/or after sintering. In other words, substantially eachproppant (e.g., over 90% or over 95% or from 95% to 99.9%) of theproppant population in a plurality of proppants have a continuouscoating around the template to form the shell and/or the shell has auniform thickness around the template (e.g., the shell thickness doesnot vary more than +/−20% in thickness, such as +/−10% or +/−5% inthickness around the entire template) and/or each proppant in theplurality of proppants is substantially flaw-free.

In one or more embodiments, the template has one or more voids in thetemplate wherein the one or more voids amount to at least 20% voidvolume (or at least 30% void volume) in the template wherein the percentis based on the entire volume of the template. The void volume can befrom 25% to 95%, or from 50% to 95%, or from 60% to 95% or more, or from70% to 90%, or from 75% to 90%).

In one or more embodiments, there can be a center void located in themiddle of the template, especially when the template is a sphere. In oneor more embodiments, the template can have a center void, but also othervoids located throughout the template.

In one or more embodiments, the shell of the present invention has theability to significantly add strength to the overall proppant even whenthe template (by itself) has a low point crush strength, such as a crushstrength of 50 psi to 1,000 psi, or from 100 psi to 500 psi, or from 100psi to 200 psi. The shell of the present invention can raise the crushstrength of the overall proppant by 50%, 100%, 200%, 300%, 400%, 500%,or more. The present invention has the ability to take surfaceimperfections existing in the template, which could cause a failure inthe proppant, and minimize or eliminate these defects in the templatesurface by forming a shell around the template. Thus, the shell of thepresent invention not only adds strength, such as crush strength to theoverall proppant, the shell of the present invention has the ability tominimize surface defects in a template, such as a template sphere.

In one or more embodiments, the template is a material that canwithstand a sintering temperature of at least 700° C. for 10 minutes inair or an oxidizing atmosphere. Preferably, the material forming thetemplate can withstand sintering at a temperature range from 80020 C. to1,700° C. However, templates which can withstand a sintering temperature(in air or an oxidizing atmosphere) of 80020 C., or 90020 C., or 1,00020C., or 1,20020 C., or 1,40020 C. for 10 minutes or other temperaturescan be used.

In one or more embodiments, any template material can be formed into asuitable template sphere for purposes of the present invention byreducing the size of the starting material, such as perlite,vermiculite, pumice, or volcanic materials by grinding, such as in anattrititor mixer and the like. The grinding will reduce the size to thedesirable size of the template as mentioned herein. Alternatively, thetemplate sphere can be formed by combining smaller particles to form acomposite particle.

In one or more embodiments, the proppant can have a substrate ortemplate that comprises an inorganic material, a metal oxide, or acombination of metal oxides and/or inorganic materials. Examplesinclude, but are not limited to, oxides of aluminum, silicon, titanium,magnesium, sodium, potassium, and the like. Alloys of these variousmetal oxides can also be used or be additionally present.

In one or more embodiments, the substrate or template can be a mineralor contain a mineral, such as containing a mineral phase. The templateor substrate, in one or more embodiments, can be or contain perlite,vermiculite, pumice, or volcanic materials that optionally areexpandable, such as expandable with the application of heat.

In one or more embodiments, the template or substrate can be a mixtureof two or more metal oxides in any proportions. For example, thetemplate or substrate can be a mixture of aluminum oxide and siliconoxide. The template or substrate can be formed into a solid sphere or ahollow sphere or a sphere having one void or multiple voids by a varietyof methods. The proportions of the metal oxides, when present as amixture, can be 1:99 to 99:1% by weight. For instance, the proportionscan be 85:15% to 70:30% or 60:40% to 27:73% by weight percent. A fluxingagent(s), such as sodium oxide, calcium oxide, magnesium oxide, lithiumoxide, and/or potassium oxide can be used as part of the templateformation, such as in amounts up to 10 wt %.

The template or substrate can be formed into a hollow sphere or a spherehaving one or multiple voids by any method, such as by a coaxial nozzlemethod, with the use of a blowing agent(s), by the solidification of thesphere from a melt phase, such as achieved with cenospheres derived fromcoal fly ash, or the aggregation of a plurality of smaller hollowspheres to form a substantially spherical agglomerate preferably withlow specific gravity.

In one or more embodiments, the template or substrate can have one void,multiple voids, a porous network of unconnected or interconnected poresor voids. The voids or pores can have substantially the same size ordiffering sizes. One or more of the voids or pores can be interconnectedand/or the voids or pores can be closed voids or pores, meaning notinterconnected with other voids or pores. The size of the voids or porescan be from about 0.05 μm to about 500 μm, such as from about 1 μm toabout 300 μm. In one or more embodiments, the template or substrate canbe highly porous, wherein 60 vol % or more of the overall volume of thetemplate or substrate is porous through voids, open voids or pores,closed voids or pores, or any combination thereof. The template orsubstrate can be porous, such that the voids and/or pores amount to 10%by volume to 95% by volume of the overall volume of the template orsubstrate, other volume percents include, but are not limited to, fromabout 30% to 95% by volume, from about 35% to about 80% by volume, fromabout 50% to about 75% by volume, wherein, as stated above, thesepercents are based upon the percent of the entire volume of the templateor substrate.

In one or more embodiments of the present invention, the substrate ortemplate can be any naturally occurring material or synthetic material.More particularly, and simply as an example, the synthetic material canbe any synthetic material that can be formed in a sphere orsubstantially spherical shape. For instance, synthetic cenospheres canbe used as the template or substrate of the present invention.

For purposes of the present invention, the template or substrate canhave one or more coatings located on a core material (composite ornon-composite material). The template or substrate can be a singleparticle or be made up of multiple particles formed, for instance, intoa composite single particle. The present invention has the ability totake various types of substrates or templates and form them intosuitable proppants that can have the proper crush strength, buoyancy,and the like. One way of transforming templates or substrates is withthe application of one or more shells on the template or substrate, suchas ceramic coatings, as described herein.

The shell in one or multiple layers can be a metal oxide or acombination of metal oxides such as oxides of aluminum, silicon,zirconium, magnesium, or any combination thereof. The shell can providea substantially higher crush strength to the overall proppant, such asexceeding 1,500 psi, such as 2,500 psi or higher or at least 5,000 psi(e.g., from above 5,000 psi to 15,000 psi). One way of achieving thisincreased crush strength is incorporating a secondary phase, one or moredopants, and/or the use of multiple layers to form the shell around thetemplate or substrate. In one or more embodiments, the shell can containreinforcing material, such as particulates, fibers, whiskers, orcombinations thereof. The reinforcing material can be present in theshell in an amount from about 1 wt % to about 25 wt %, and moreparticularly from about 5 wt % to about 15 wt %, based on the weightpercent of the shell. The particulates or fibers typically can have asize of from about 0.1 μm to about 5 μm, more particularly from about 1μm to about 3 μm. The reinforcing material can be uniformly distributedthroughout the surface area of the shell. Examples of particularreinforcing materials include, but are not limited to, carbon black,fiberglass, carbon fibers, ceramic whiskers, ceramic particulates,and/or metallic particles. The shell can contain secondary phases, suchas, but are not limited to, inorganic or ceramic phases. Examplesinclude metal oxide(s), metal carbide(s), metal nitride(s), or anycombination thereof. Zirconium oxide, zirconium carbide, and zirconiumnitride are examples. The zirconium oxides (zirconium carbides, and/orzirconium nitrides) can be stabilized in a useful crystallographicphase, such as through the use of one or more elements, such as metals.For instance, zirconium oxide, such as a tetragonal phase of zirconiumoxide, can be stabilized through the additions of the oxides ofmagnesium, calcium, cerium, yttrium, scandium, or any combinationthereof. The carbides or nitrides of zirconium can be stabilized throughthe use of silicon, titanium, tungsten, aluminum, boron, or anycombination thereof. The stabilizers can be present in any amount, suchas from about 3.5 wt % to about 5.5 wt % based on the weight of metaloxide, such as zirconium oxide. Other examples of amounts include fromabout 10 wt % to about 17 wt %, based on the weight of metal oxide, suchas zirconium oxide. The formation of the metal carbides, such ascarbides of silicon, titanium, zirconium, tungsten, aluminum or boron(optionally with the one or more stabilizers), can be formed underelevated temperatures (such as temperatures from about 500° C. to about1,20020 C.) in an atmosphere of carbon monoxide, where silicon,titanium, zirconium, tungsten, aluminum, or boron (or other elementalparticles) can come in contact with carbon to form the carbide phase,for instance, zirconium carbide. An unsintered shell can be coated witha carbon containing material (such as carbon black, pitch, charcoal, orcoke derived from coal) prior to heat treating to form a carbide.

Similarly, the metal nitrides (e.g., zirconium nitride) (having one ormore stabilizers) can be formed under elevated temperatures in anatmosphere containing ammonia, nitrogen, nitrous oxide, or anycombination thereof. Examples of elevated temperatures include, but arenot limited to, from about 500° C. to about 1,20020 C.

In one or more embodiments, the shell can contain one or more dopants,such as materials to improve the densification of the shell material,retard the densification of the shell material, and/or to improve thesusceptibility of the shell material to external influences during thesintering process.

In one or more embodiments, the shell can be surface modified, forinstance, by the addition of one or more inorganic materials or phases,or the attachment (e.g., chemical attachment or bonding) of one or morechemical groups, such as hydrophilic groups or hydrophobic groups. Thechemical groups can be surfactants, polymers, ionic groups, ionizablegroups, acid groups, salts, surface active agents, and the like. Thesurface modification can improve the surface morphology of the proppant,especially after the proppant is a sintered proppant. The inorganicmaterial or phases used for surface modification can include glassymaterials, such as silicon oxide, alone or with the addition of oxidesof sodium, potassium, calcium, zirconium, aluminum, lithium, iron, orany combination thereof. The amount of the silicon oxide can be fromabout 70 wt % to about 99 wt %, such as from about 85 wt % to about 95wt %, and the addition of the one or more other oxides, such as sodiumoxide and the like, can be from about 1 wt % to about 15 wt %, such asfrom about 2 wt % to about 10 wt %. The surface modification of theshell can include the application of one or more organic materials(e.g., aliphatic compounds, ionic compounds, surfactants, aromaticcompounds, polymeric compounds) or the application of an organicphase(s). The organic material or chemical groups can be bonded to theshell surface or adsorbed, or absorbed or otherwise attached. Theorganic material or organic phase can modify the proppant's propensityto interact with aqueous solutions, thus making the proppant eitherhydrophobic, hydrophilic, or hydro-neutral. The surface modification ofthe shell can include the use of substances that are effectivelyactivated by temperature elevation of the proppant to yield amodification of the proppant transport fluid (e.g., breaking the gelused to transport the proppant through the subterranean regions). Thesurface treating, which can occur after sintering of the proppant, canhave the ability to improve one or more chemical and/or mechanicalproperties, such as enhanced transportability.

The proppants of the present invention can be designed to improvereservoir flow rates by changing the hydrophilic properties of theproppants themselves. The hydrophilic nature of current proppants causeswater to be trapped in the pore spaces between proppants. If this watercould be removed, flow rates would be increased. A route to theprevention of this entrapped water within the proppant pack, is tofunctionalize the surface of the proppant particles with at least onechemical substituent such that more hydro-neutral or hydrophobic surfacewetting properties of the particles are achieved. Such chemicalsubstituents include, but are not limited to, functionalizedcarboxylate, phosphate, or esters, where the functionalized group can bean alkyl group, or other moiety conferring a degree of surfacehydrophobicity or hydrophiilicity. In particular, the surface can befunctionalized with hydrophobic carboxylic acids such as hexanoic acidor parahydroxybenzoic acid, or methoxy, methoxy(ethoxy), ormethoxy(ethoxyethoxy) acetic acids. Furthermore, the carboxylatealumoxanes of each of these acids may also be used for the surfacefunctionalization. The chemical groups mentioned throughout can be usedas well for functionalization. These functionalizations of the proppantsurface allow for varying of the wetting properties continuously withinthe range and spectrum of hydrophilicity, hydro-neutrality, andhydrophobicity of the surface.

In one or more embodiments of the present invention, the shell or one ormore layers comprising the shell can be densified by a variety ofmethods. Examples of methods that can be used to densify the shell or alayer of the shell include, but are not limited to, liquid phasesintering, reactive phase sintering, and/or solid state sintering. As amore specific example, the densification can be achieved by indirectradiant heating, direct infrared radiation, direct conduction of theheat flux from the environment to the proppant shell, excitation of theconstituent molecules of the shell, and consequent heating of the shellby electromagnetic radiation, inductive coupling of the shell materialto an external excitation field of alternating current for instance,with a frequency from 5 to 1,000 HZ. The heating of the shell byelectromagnetic radiation can be at a frequency from 2 to 60 GHZ, suchas that generated by a magnetron. The pressure assisted sintering can becarried out with an application of external gas pressure to the systemduring heat treatment, with pressures, for instance, ranging fromambient to 1,500 PSIG.

For purposes of liquid phase sintering, additives may be used, such asmetal oxides. Examples include, but are not limited to, oxides ofaluminum, silicon, magnesium, titanium, lithium, sodium, calcium,potassium, or any combination thereof. The amount of the additives canbe from about 0.1 wt % to about 5 wt %, such as from about 0.25 wt % toabout 2 wt %, per weight of oxide in the shell. The liquid phasesintering additives can be introduced by way of chemical mixing,coating, or dissociation of a secondary phase in the coating.

With respect to the secondary phases which may be present with theshell, the secondary phases can be achieved by forming the secondaryphase in-situ with the other ingredients used to form the coating, whichultimately forms the shell on the template or substrate. The secondaryphase can be prepared separately and then added to the composition usedto form the shell by blending or other introduction techniques.

In one or more embodiments, the shell can comprise multiple layers, suchas two or more layers, such as two, three, four, or five layers. Thelayers can be the same or different from each other. Various layers canbe used to achieve different purposes or physical or chemicalproperties. For instance, one layer can be used to improve crushstrength, and another layer can be used to achieve buoyancy. For examplethe use of an aluminosilicate material as a layer material would producean interfacial layer between the template and subsequent layers toimprove boding of the layers. The use of an aluminosilicate layer, byvirtue of its lower elastic modulus, would also improve the mechanicalproperties of the system due the ability to inhibit crack propagation.Examples of such a material include, mullite, cordierite, and dopedaluminosilicates with a dopant phase such as, but not limited to,lithium, magnesium, sodium. The lower specific gravity of the layer canreduce the overall specific gravity of the proppant, thus increasing thebuoyancy of the proppant. The load bearing layer can be any of thewidely available engineering ceramics such as, but not limited toaluminum oxide, stabilized zirconium oxide, carbides and nitrides. Byusing different layers, one or more synergistic results can be achievedwith respect to the performance of the proppant. For instance, the useof two or more layers, which may be the same or different, can achievesynergistic improvement with respect to the generation of useful strainfields in the system, beneficial crack tip influences, or a combinationof these properties. The thermal expansion coefficient mismatch betweenlayers may generate residual compressive strain fields in one or more ofthe layers leading to apparent increases in fracture toughness andstrength of the overall proppant system. The presence of an interfacebetween layers will lead to deviation and/or trapping of an advancingcracktip during loading to improve the apparent fracture toughness andstrength of the overall proppant system.

Besides the various techniques already mentioned, the shell can beformed on the template or substrate by a fluidized bed method. Forinstance, the shell can be formed through the use of heated fluidizingair, dried fluidizing air at ambient temperature, or dried heatfluidizing air. The ambient temperatures would be from 15 to 3020 C. andthe heated fluidizing air would be from about 50 to about 200° C. Thedew point of the fluidizing air can be below 0° C. The temperature ofthe fluidizing air can be from about 25° C. to about 20020 C. The volumeof the fluidizing air admitted into the chamber can be the same orvaried to effect adequate fluidization of the templates during thecoating process. For example, as the coating builds, the mass of theindividual particles increases such that they may no longer remainadequately fluidized, thus an increase in air velocity (volume) can beused to maintain the particles in suspension. The material forming theshell can be applied to the templates in the fluidizing bed through theuse of a spray nozzle. The spray nozzle can have both single and/or dualfluid designs. The single fluid design can effect the atomization of thesolution that forms the shell through, for instance, the effects ofpressure reduction at the orifice of the nozzle. The dual fluid nozzlecan effect the atomization of the solution that forms the shell throughthe addition of atomizing air to the solution either prior to exitingthe nozzle or after exiting the nozzle, i.e., internal mix or externalmix design.

In one or more embodiments, the template or substrate can be modified,such as surface modified, through a variety of techniques. For instance,the surface of the template or substrate can be modified through one ormore heat treatments prior to the formation of the shell on thetemplate. Another form of surface treatment can be to modify chemicallythe surface of the template, such as by glazing, application of a bondcoat, or chemical etching. The chemical modification can improve theperformance and/or stabilize the template surface. Removal of residualimpurities, cleaning of the surface, reduction of residual staindistributions prior to coating, increasing the microscopic roughness ofthe surface to improve coating bond strength or removing and/orimproving the morphology of protuberances. Furthermore, the modificationcan be achieved by the preferential removal of one or more constituentphases on the template. For example, a wash with caustic soda maypreferentially dissolve silica from the template.

Examples of templates or substrates that can be used in the presentinvention include metal-fly ash composites, which can contain metaland/or metal alloys. Examples include those set forth in U.S. Pat. No.5,899,256, incorporated in its entirety by reference herein. The metalor metal alloy can be aluminum or an aluminum alloy. Further, themetal-fly ash composites contain cenosphere fly ash particles or otherfly ash particles. The template or substrate used in the presentinvention can be particles, such as spheroidal particles of slag and/orash. These particles, for instance, can contain SiO₂, Al₂O₃, and/or CaO.Other oxides can be present, such as other metal oxides. Examples ofparticles can include those set forth in U.S. Pat. No. 6,746,636, whichis incorporated in its entirety by reference herein.

In one or more embodiments of the present invention, the template orsubstrate can be a sintered composite particulate containingnanoparticles and/or a clay material, bauxite, alumina, silica, ormixtures thereof. The nanoparticles can be hollow microspheres, such ashollow mineral glass spheres, such as SPHERELITES, CENOLIGHTS, SCOTCHLIGHT, or Z-LIGHT SPHERES. The template or substrate can be formed ofclay or hydrided aluminum silicate, bauxite, containing 30 to 75% Al₂O₃,9 to 31% H₂O, 3 to 25% FeO₃, 2 to 9% SiO₂, and 1 to 3% TiO₂.

The template or substrate (or the shell) can have a resin or polymercoating on it to aid in dispersibility, to aid in receiving a shell ofthe present invention, or for processing reasons. The template orsubstrate can contain nanoparticles, such as nanoclays, carbonnanofibers, silica, carbon nanotubes, nanoparticle minerals, such assilica, alumina, mica, graphite, carbon black, fumed carbon, fly ash,glass nanospheres, ceramic nanospheres, or any combination thereof. Thenanoparticles can have a size of 10 nanometers to 500 nanometers. Thetemplate or substrate can be natural sand, quartz sand, particulategarnet, glass, nylon pellets, carbon composites, natural or syntheticpolymers, porous silica, alumina spheroids, resin beads, and the like.It is to be understood that the term “template or substrate, ” as usedthroughout the application, includes template spheres, and preferablyhaving at least one void.

In one or more embodiments, the template or substrate can be a porousparticulate material, which can be treated with a non-porous coating orglazing material. For instance, the porous particulate material can beporous ceramic particles, such as those set forth in U.S. Pat. No.5,188,175, incorporated in its entirety by reference herein. Thenon-porous coating or glazing material can be a resin or plastic.

In one or more embodiments, the template or substrate can be syntheticmicrospheres, such as ones having a low alkali metal oxide content.Examples include those set forth in U.S. Patent Application PublicationNo. 2004/0079260, incorporated in its entirety by reference herein. Thetemplate or substrate can be spherical material made from kaolin clayhaving an aluminum content distributed throughout the pellet, such asthose described in U.S. Patent Application Publication No. 2004/0069490.

In one or more embodiments, the template or substrate can be sintered,spherical composite or granulated pellets or particles, such as onescontaining one or more clays as a major component and bauxite, aluminum,or mixtures thereof. The pellets can have an alumina-silica ratio fromabout 9:1 to about 1:1 by weight.

In one or more embodiments, the template or substrate can be porousceramic particles having an average particle diameter of 50-2,000microns. The porous ceramic particles can have pores averaging fromabout 1 to 100 microns in diameter, and other pores averaging from about0.001 to 1 micron in diameter. The ceramic material can include alumina,silica, zirconia, titania, zirconium aluminum titanate nitrides,carbides, or mixtures thereof. The particles can have an average surfacearea (BET) of about 1 to 50 m²/g. The particles can have an averagesurface area (BET) of from about 100 to 500 m²/g.

In one or more embodiments, the shell comprises one or more layers. Whenmultiple layers comprise the shell, the layers can be the same ordifferent from each other. The thickness of each layer can be the sameor different. As stated, one or more layers can comprise an inorganic orceramic material, such as alumina or other ceramic or inorganicmaterials as described herein. One or more of the layers can be atoughened layer, such as a toughened inorganic or ceramic layer, such asalumina. For instance, the ceramic or inorganic layer that comprises atleast one layer of the shell, can be strengthened or toughened by theaddition or presence of metal oxide, metal nitrides, and/or metalcarbides, and the like (e.g., zirconium oxide, zirconium nitride, and/orzirconium carbide). The strengthened layer can be achieved by eitheradding an additional layer comprising an inorganic or ceramic material,such as metal oxide, metal nitrides, or metal carbides, or in thealternative or in addition, the previously-applied layer containing theinorganic or ceramic material can be converted or chemically altered tocontain a nitride, a carbide, or the like. Either option is possible.The nitrides can contain or be a nitride of Si, Al, Ti, Bo, and thelike. The carbides can contain or be a carbide of Si, Al, Ti, Bo, andthe like.

In one or more embodiments, the shell can comprise one or more layers,and can provide a surface repairing of the template surface, such as atemplate sphere surface. The surface repair can be achieved, forinstance, by providing a glazing layer on the template surface. Theglazing layer can at least partially infiltrate or penetrate below thetop or outer surface of the template surface, such as in cracks or flawsin the template sphere. For instance, a thin layer (e.g., 0.5 micron to10 microns) of silica, mullite, cordierite, spodumene, or otherinorganic, mineral-containing, or ceramic-containing materials. Thisglazing layer can form part of the overall shell and serve as one layerof the shell. Preferably, this surface repairing of the template isachieved by glazing the immediate surface of the template. Another formof repairing the surface of the template prior to formation of the shellon the template surface can be achieved by heat treatment, which candensify, consolidate, or otherwise repair cracks or other flaws in thesurface of the template. The heat treatment can occur at any temperaturedepending upon the composition of the template material, for instance,at temperatures of from about 500° C. to 1,700° C. for a time sufficientto surface repair (e.g., 10 minutes or more). In addition, as anotheroption to surface repairing of the template surface, the cracks or flawscan be infiltrated with suspensions of alumoxanes or other inorganic(e.g., mullite, metal oxides) or ceramic-containing materials, such asalkyl-alumoxanes, methyl-alumoxanes, and the like. Further, dopantscontaining one or more of the alumoxanes or other materials can be used.Also, alumina-containing slurries and mullite-containing slurries, aswell as inorganic or ceramic-containing slurries, can be used.Preferably, the particle size of the alumoxane, alumina, mullite, orother inorganic or ceramic-containing particles are small enough to fitwithin the cracked surface, such as particles on the nano-scale level,for instance, from 1 nanometer to 1,000 nanometers.

In one or more embodiments, one of the layers comprising the shell canbe a resin or polymer-containing layer. For instance, the resin orpolymer-containing layer can be the outermost layer of the shell and canoptionally have a tacky surface, which permits the proppant to have theability to remain in the subterranean formation during hydrocarbonrecovery. Also, should the proppant have a structure failure, an outerresin coating, such as one that is tacky, can permit the failure of theproppant to stay in the proppant location in the subterranean formationwithout interfering with hydrocarbon recovery. The resin coating orpolymer layer can be any thickness as described herein with respect toany other layer of the shell, such as from about 5 microns to 150microns. The resin or polymer layer can be located anywhere as part ofthe shell, such as the innermost layer of the shell, the outermost layerof the shell, or one of the intermediate layers of the shell dependingupon the purpose of the resin or polymer layer.

In one or more embodiments, the template material or sphere can be orcontain a geopolymer or contain a pore forming or pore containingmaterial (e.g., dissolvable or decomposable material) which can besubsequently subjected to chemical etching or a burn-out, whereinportions of the geopolymer or other pore-forming material can beremoved.

In one embodiment, the template, such as the template sphere, cancomprise ceramic material or inorganic material, which can be presentwith pore or void-forming material, wherein the pore or void-formingmaterial is removable by any process, such as chemical etching, heating,and the like. Once the pore-forming or void-forming material ispartially or completely removed, the template sphere or material isachieved, which has the desirable void volume percent as describedherein.

In one or more embodiments of the present invention, the shell can beformed on the template, such as the template sphere, while the templateor template sphere is in a green state, and then the entire proppanthaving the template and the shell can then be subjected to sintering orheat treatment to form or consolidate the shell and consolidate orcalcine the template sphere as well.

In one or more embodiments, the template material, such as the templatesphere, can have a specific gravity of from 0.25 to 0.85 g/cc. Thisspecific gravity for the template sphere can be especially useful whencoating the template sphere to form the shell, especially when thecoating is being achieved with the use of a fluidized bed.

The template sphere or material is preferably a particle not formed bygranulation or agglomeration techniques, but is a single individualcontinuous particle having one or more voids.

In one or more embodiments, the template or substrate can be a hollowmicrosphere made by the coaxial process described in U.S. Pat. No.5,225,123, incorporated in its entirety by reference herein. Thisprocess includes the use of feeding a dispersed composition in a blowinggas to a coaxial blowing nozzle, wherein the coaxial blowing nozzle hasan intercoaxial nozzle for blowing gas and an outer nozzle for thedispersed particle composition. The hollow microspheres can be made fromceramic materials, glass materials, metal materials, metal glassmaterials, plastic materials, macroparticles, and the like.

The proppants of the present invention can be used in a variety ofareas. The proppants can be used as substrates as semi-permeablemembranes in processes for carrying out gas and liquid separations andfor use as substrates for catalysts and enzymes. The proppants can beused in processes for the manufacture and purification of pharmaceuticalor chemical products, for instance, using or derived fromgenetically-engineered bacteria, natural living organisms, and enzymes.The proppants of the present invention can be used as containers forliquids, adsorbents, absorbents, or catalysts or as containers forchemical agents whose release is subject to predetermined control (e.g.,controlled slow release).

The proppants of the present invention can be used in one or more of thefollowing areas as a composition, an additive, and/or to fully replaceor partially replace the filler or reinforcing agent conventionallyused, using similar or the same amounts, or lesser amounts, to achievethe same or improved properties: proppants for oil and gas industry,lightweight high strength fillers for polymers, syntactic foams foraerospace applications, high performance fillers for cement andconcrete, high performance refractory materials, high strength,lightweight insulating materials, carriers for catalysis systems, watertreatment systems, high strength, lightweight particulate reinforcementsfor polymer matrix composites, high strength, lightweight particulatereinforcements for ceramic matric composites, high strength, lightweightparticulate reinforcements for metal matrix composites, high performancecasting sand for metal casting applications, or friction reducingfillers for polymer processing systems (e.g. extrusion, die casting,etc).

The proppant of the present invention can be prepared by forming atemplate sphere and providing a shell around the entire outer surface ofthe template sphere, and then sintering the shell to form a continuoussintered shell. The sintering can be liquid phase sintering, reactivephase sintering, or solid state sintering, or any combination thereof.The sintering can comprise indirect radiant heating, direct infraredradiation, direct conduction of heat flux from an environment to saidproppant, excitation of molecules of said shell, and consequent heatingof said shell by electromagnetic radiation, or inductive coupling of theshell to an external excitation field of alternating current. Thetemplate sphere can be formed by various processes which preferably makea template sphere having one or more voids. The template sphere can beformed by a spray drying process, a dehydrating gel process, a sol-gelprocess, a sol-gel-vibrational dropping process, a drop tower process, afluidized bed process, a coaxial nozzle gas-bubble process, athermolysis process, a chemical etching process, or a blowing process.Examples of these various processes are set forth below, and patents andapplications providing details of these processes which can be adaptedto the present invention are further provided. below. It is noted thatas an option the step of solidifying, hardening, or sintering to formthe final template sphere can be optional, and the template sphere canbe left in a green state so that upon the formation or densification ofthe shell by sintering, the green template sphere can be also sintered,hardened, or otherwise solidified to form the proppant of the presentinvention.

The hollow template spheres can be made from aqueous or non-aqueoussuspensions or dispersions of finely divided inorganic or organic solidparticles, such as ceramic, glass, metal and metal glass particles,having particle diameters in the range of from about 0.01 to 10 microns(μm), a binder material, a film stabilizing agent, a dispersing agentfor the solid particles, and a continuous aqueous or non-aqueous liquidphase. The suspension or dispersion is blown into spheres using acoaxial blowing nozzle, and the spheres are heated to evaporate thesolvent and further heated or cooled to harden the spheres. The hardenedspheres are then subjected to elevated temperatures to decompose andremove the binder and any residual solvent or low boiling or meltingmaterials. The resulting porous hollow spheres are then fired at furtherelevated temperatures to cause the particles to sinter and/or fuse atthe points of contact of the particles with each other such that theparticles coalesce to form a strong rigid network (lattice structure) ofthe sintered-together particles.

A coaxial blowing nozzle and a blowing gas to blow hollow spheres from acontinuous liquid phase and dispersed particle film forming compositioncan be used and can comprise feeding the blowing gas to an inner coaxialnozzle, feeding the dispersed particle film forming composition to anouter coaxial nozzle, forming spherically shaped hollow spheres in theregion of the orifice of the coaxial blowing nozzle and removing thehollow spheres from the region of the orifice of the coaxial blowingnozzle. A transverse jet entraining fluid can be used to assist in thesphere formation and the detaching of the hollow spheres from theblowing nozzle. The continuous liquid phase of the dispersed particlefilm forming composition allows the hollow spheres to be blown byforming a stable film to contain the blowing gas while the hollow sphereis being blown and formed. The dispersed particles in the dispersedparticle composition, as the dispersed particle composition is formingthe hollow sphere and after the sphere is formed, link up with eachother to form a rigid or relatively rigid lattice work of dispersedparticles which dispersed particle lattice work with the binder andcontinuous liquid phase comprise the hollow green spheres. The hollowspheres, after they are formed, can be hardened in ambient atmosphere orby heating and removing a portion of the continuous phase. The hardenedhollow green spheres have sufficient strength for handling and furthertreatment without significant breaking or deforming of the microspheres.

The hardened green spheres can be treated at elevated temperatures toremove the remainder of the continuous liquid phase and volatilematerials such as binder, film stabilizing agent and dispersing agent.The treatment at elevated temperatures sinters and coalesces thedispersed solid particles to form rigid hollow porous spheres that canbe substantially spherical in shape, can have substantially uniformdiameters and can have substantially uniform wall thickness. The heatingat elevated temperatures, in removing the continuous phase and addedmaterials, creates interconnecting voids in the walls of the sphereswhich result in the porous characteristics of the spheres. The sinteringand coalescing of the dispersed solid particles, depending on the timeand temperature of the heating step, can cause a small degree ofcompaction of the dispersed particles and can cause the coalescing ofthe particles at the points in which they are in contact to form rigid,uniform size and shaped spheres of uniform wall thickness, uniform voidcontent and uniform distribution of voids in the walls and highstrength. Because the porosity is a result of the removal of thecontinuous phase from uniformly dispersed solid particles, the pores canbe continuous from the outer wall surface of the template sphere to theinner wall surface of the template sphere and the walls of the templatespheres can have substantially uniform void content and uniformdistribution of the voids that are created.

The hollow template spheres in general can be substantially spherical,have substantially uniform diameters, and have substantially uniformwall thickness and the walls have uniform void content and voiddistribution and voids which are connected to each other and to theinner and outer sphere wall surfaces. The walls of the hollow porousspheres can be free of latent solid or liquid blowing gas materials, andcan be substantially free of relatively thinned wall portions orsections and bubbles.

The hollow spheres can be made from a wide variety of film formingdispersed particle compositions, particularly dispersed ceramic, glass,metal, metal glass and plastic particle compositions and mixturesthereof. The dispersed particle compositions can comprise an aqueous ornonaqueous continous liquid phase and have the necessary viscositieswhen being blown to form stable films. The hollow sphere stable filmwall after the sphere is formed rapidly changes from liquid to solid toform hollow green spheres. The hollow green spheres can be substantiallyspherical in shape and can be substantially uniform in diameter and wallthickness.

The hollow green spheres as they are being formed and/or after they areformed can have a portion of the continuous liquid phase removed fromthe dispersed particle composition from which the spheres were formed.The removal of continuous liquid phase can act to bring the dispersedparticles closer together and into point to point contact with eachother. The dispersed particles can then link up with each other to forma rigid or relatively rigid lattice work of dispersed particles whichparticles lattice work with the binder (if one is used) and continuousliquid phase (that remains) comprise the hollow green spheres. Thehollow green spheres are free of any latent solid or liquid blowing gasmaterials or latent blowing gases. The walls of the hollow green spheresare free or substantially free of any holes, relatively thinned wallportions or sections, trapped gas bubbles, or sufficient amounts ofdissolved gases to form bubbles. The term “latent ” as applied to latentsolid or liquid blowing gas materials or latent blowing gases is arecognized term of art. The term “latent ” in this context refers toblowing agents that are present in or added to glass, metal and plasticparticles. The glass, metal and plastic particles containing the “latentblowing agent ” can be subsequently heated to vaporize and/or expand thelatent blowing agent to blow or “puff” the glass, metal or plasticparticles to form spheres. The hollow green spheres can have walls thatare substantially free of any holes, thinned sections, trapped gasbubbles, and/or sufficient amounts of dissolved gases to form trappedbubbles.

In general, the hollow template spheres can contain a single centralcavity, i.e. the single cavity is free of multiple wall or cellularstructures. The walls of the hollow spheres can be free of bubbles, e.g.foam sections. The hollow template spheres can be made in variousdiameters and wall thickness. The spheres can have an outer diameter of200 to 10,000 microns, preferably 500 to 6000 microns and morepreferably 1000 to 4000 microns. The spheres can have a wall thicknessof 1.0 to 1000 microns, preferably 5.0 to 400 microns and morepreferably 10 to 100 microns. When the dispersed particles are sintered,the smaller particles can be dissolved into the larger particles. Thesintered particles in the hollow porous spheres can be generally regularin shape and have a size of 0.1 to 60 microns, preferably 0.5 to 20microns, and more preferably 1 to 10 microns.

In certain embodiments of the invention, the ratio of the diameter tothe wall thickness, and the conditions of firing and sintering thehollow template spheres can be selected such that the spheres areflexible, i.e., can be deformed a slight degree under pressure withoutbreaking. The preferred embodiment of the invention, particularly withthe ceramic materials, is to select the ratio of the diameter to wallthickness and the conditions of firing and sintering the hollow porousspheres such that rigid hollow porous spheres are obtained.

Another process to make the template sphere can involve a dehydratinggel process. An example of such a process can include the steps ofadding percursor material comprising an aqueous solution, dispersion orsol of one or more metal oxides (or compounds calcinable to metal oxide)to a liquid body of a dehydrating agent comprising an organicdehydrating liquid, agitating the liquid body to maintain the resultingdroplets of the precursor material in suspension and prevent settlingthereof, to maintain relatively anhydrous dehydrating liquid in contactwith the surface of the droplets as they are dehydrated, and to rapidlyextract within 30 seconds at ambient temperatures of 20° to 40° C., themajor amount of water from said droplets and form gelled microparticlestherefrom, the predominant amount of said gelled microparticles being inthe form of spherical, gelled, porous, liquid-filled spheres, recoveringsaid liquid-filled spheres, drying the resulting recovered spheres attemperatures and pressures adjusted to minimize fracture and burstingthe same and remove liquid from within the recovered spheres, and firingthe resulting dried spheres to form spherical ceramic spheres theperipheral wall or shell of each which encloses the single hollow withinthe interior thereof being porous and heat-sealable, homogeneous, andmade of non-vitreous ceramic comprising polycrystalline metal oxide oramorphous metal oxide convertible to polycrystalline metal oxide uponfiring at higher temperature.

Another method to make the template sphere can be a thermolysis process,such as the process described in U.S. Pat. No. 4,111,713, the disclosureof which is incorporated herein by reference. The method involves thepreparation of hollow spheres, the exterior wall of which comprises athermally fugitive binder material and sinterable inorganic particlesdispersed in the binder material. By “thermally fugitive ” is meantmaterials that upon heating of the spheres will be removed from thespheres, e.g., by vaporization and/or oxidation or burning. Forinstance, at least 20% by volume of the thermally fugitive material canbe removed, or from 20% to 100% or 70% to 99% by volume. Natural orsynthetic organic materials which are readily burned such as corn starchsyrup, phenolic resins, acrylics and the like can be used as bindermaterials.

Besides a binder material, the solidifiable liquid sphere includes avolatile void-forming agent such as taught in U.S. Pat. No. 4,111,713.Other ingredients may also be included, such as a solvent or otherdispersing liquid. In addition, a metal or other inorganic material maybe included. Metallic binder combinations can be obtained by using (1) acolloidal dispersion of a metal, metalloid, metal oxide, or metal saltor (2) a metal, metalloid, metal oxide, or metal salt dispersion in aphenolic resin or other organic binder.

Typically the solidifiable liquid sphere is formed at room temperature,e.g., by dissolving the binder material in a solvent or dispersing it inanother liquid. However, solid granules of binder material that becomeliquid during the tumbling operation may also be used.

During the sphere-forming operation the binder material should achieve aviscosity that is low enough for the parting agent particles to bewetted by the spheres, and preferably low enough so that any cellsforming inside an evacuated sphere will tend to at least partiallycoalesce, whereby binder material will be concentrated at the exteriorspherical wall or shell of the sphere. The parting agents can be appliedby a mixer or by a fluidized bed, using a similar approach that isdescribed in forming the shell. At the same time the viscosity of thebinder material should be high enough so that the expanded sphere willnot deform excessively while sphere formation is taking place. Theuseful range of viscosities for the binder material is broad, rangingfrom at least about 50 to 100,000 centipoises, but an especiallypreferred range is between about 100 and 10,000 centipoises. The spheresof binder material in the tumbling, sphere-forming operation are termedliquid herein, since even when at high viscosity they are flowable. Therange of useful viscosities will vary with particle size and the easewith which the parting agent particles can be wet. Surfactants can beused to advantage either as an ingredient in the binder material or as atreatment on the parting agent particle.

The parting agent particles used in practicing the invention can besolid discrete free-flowing particulate material which is sufficientlyinert, including sufficiently nonmelting, during the sphere-formingoperation to retain a parting function. In addition, parting agents thateventually become the primary or only constitutent of the sphere wallsshould be sinterable inorganic materials. Suitable metal parting agentsare iron, copper, nickel and the like. Suitable metalloid parting agentsinclude carbides such as silicon carbide, nitrides such as boronnitride, borides, suicides and sulfides. Suitable metal oxide partingagent particles include alumina, zirconia, magnetite, silica, mullite,magnesite, spinels and the like. Suitable metal salt parting agentparticles include metal hydroxides, nitrates, and carbonates.

Mixtures of different parting agent particles are used in someembodiments of the invention. For example, parting agent particlesproviding better flow properties, e.g., spheres, which may or may not besinterable, may be mixed with irregular sinterable parting agentparticles. Alternatively, mixtures are used to provide pigmentation,flame-retardancy, or variety in physical properties of the final sphere.However, sinterable particles generally constitute at least a majority,and preferably at least 60 volume percent, of the exterior wall of asphere so as to obtain adequate coherency and strength.

Generally the parting agent particles can range from a few micrometersup to several hundred micrometers in size. They generally have adiameter no larger than the thickness of the wall of the final hollowsphere.

Generally the solidifiable liquid spheres are used in sizes that producehollow spheres about ½ millimeter to 2 centimeters in diameter. Spheresof the invention can be made with good uniformity of sizes by usingbinder material granules or spheres of uniform size. Further, of course,hollow spheres may be screened after formation to provide desired rangesof size.

The template spheres are generally round but need not be perfectlyspherical; they may be cratered or ellipsoidal, for example. Suchirregular, though generally round or spherical, hollow products areregarded as “spheres ” herein.

The hollow template spheres can have a single hollow interior space,such as described in U.S. Pat. No. 4,111,713. The interior space in thesphere may be divided into a number of cells by interior walls havingessentially the same composition as the exterior wall; but even suchspheres have an outer wall, usually of rather constant thickness and ofgreater density, around the interior space. The outer wall is continuousand seamless (that is, without the junction lines resulting when twoseparately molded hemispheres are bonded together), though the wall maybe permeable or porous. The thickness of the outer wall is generallyless than about ½ the radius of the sphere and may be quite thin, asthin as 1/50 the radius, for example.

Another method to form the template sphere is also a dehydrating gel orliquid method which uses an aqueous precursor material that contains anaqueous solution, dispersion or sol of one or more metal oxides or metalcompounds calcinable to metal oxides, or mixtures of said forms ofprecursor materials. The precursor material should be pourable andstable, that is, non-gelled, non-flocculated or non-precipitated. Theequivalent concentration of the metal oxide in the precursor materialcan vary widely, e.g. a few tenths of one weight % to 40 or 50 weight %,and the particular concentration chosen will be dependent on theparticular form of the precursor metal oxide and dehydrating liquid usedand the desired dimensions and proposed utility of the template spheres.Generally, this concentration will be that sufficient to promote rapidformation of droplets in the dehydrating liquid and, generally, thelower the equivalent concentration of metal oxide in the precursormaterials, the thinner the walls and the smaller the diameters of thespheres.

The dehydrating liquid used to dehydratively gel the precursor materialis preferably a liquid in which water has a limited solubility and inwhich water is miscible to a limited extent. Such a dehydrating liquidwill practically instaneously cause formation of liquid droplets of theprecursor material and rapidly extract the major amount of the waterfrom the droplets to form discrete, dispersed, liquid-filled sphereshaving a porous gelled wall or shell, the physical integrity of which ismaintained in the body of dehydrating liquid. The formation of asubstantially quantitative yield of gelled spheres can be completewithin 30 seconds. Further, this formation does not require heating(i.e., it can be accomplished at ambient room temperature, e.g., 2320C.) nor does it require use of a barrier liquid. Though a small amountof solid beads may also be formed, the predominant amount, i.e., atleast 85-95% or higher, of the microparticles formed will be in the formof template spheres. If the liquid-liquid extraction is carried out in abatch operation, there may be a tendency to form said small amount ofsolid beads (or relatively thicker-walled microcapsules) toward the endof the extraction due to the progressively decreasing dehydratingability of the dehydrating liquid as it extracts the water from theprecursor material.

Generally, dehydrating liquids can have a limited solubility of about 3to 50 weight %, preferably 15 to 40 weight % for water (based on theweight of the dehydrating liquid) at 23° C. Representative organicdehydrating liquids useful are alcohols, such as alkanols with 3-6carbon atoms, e.g. n-butanol, sec-butanol, 1-pentanol, 2-pentanol,3-methyl-2-butanol, 2-methyl-2-butanol, 3-methyl-3-pentanol,2-methyl-1-propanol, 2,3-dimethyl-2-butanol and 2-methyl-2-pentanol,cyclohexanol, ketones such as methyl ethyl ketone, amines such asdipropylamine, and esters such as methylacetate, and mixtures thereof.Some of these dehydrating liquids, e.g. n-butanol, when used to formspheres with relatively large diameters, e.g. 100-500 microns or larger,may have a tendency to cause micro-cracks in the walls of the spheres.Such micro-cracks can be prevented or minimized when such dehydratingliquids are used to form large spheres by adding a small amount of waterto such dehydrating liquids, e.g. 5 to 10% by weight of the dehydratingliquid. However, the resulting water-dehydrating liquid mixture stillhas said limited solubility for water, preferably at least 15 weight %.

The liquid-liquid extraction step can be carried out at ambienttemperatures, e.g. 20°to 40° C.; higher temperatures, e.g. 60° C. andhigher, cause fragmentation of the gelled spheres. Excellent,substantial, quantitative yields, e.g. 95% and higher, of gelledspheres, based on the equivalent oxide solids content of the precursormaterial, can be conveniently achieved at room temperature (23° C.). Inorder to quickly and efficiently dehydratively gel the droplets of theprecursor material in a batch operation, the body of dehydrating liquidcan be subjected to externally applied agitation (e.g. by swirling thebody of dehydrating liquid or by inserting a stirrer therein) when theprecursor material is added thereto, and said agitation is continuedduring the course of dehydration of the resultant droplets of precursormaterial. This agitation maintains the droplets in suspension (andthereby prevents agglomeration and settling of the droplets) and ensuresmaintenance of relatively anhydrous dehydrating liquid in contact withthe surface of the droplets as they are dehydrated. In a continuousliquid-liquid extraction operation, equivalent agitation can beaccomplished by adding the precursor material at a point to a stream ofthe dehydrating liquid flowing at a sufficient rate to maintain thedroplets in suspension in the course of their dehydration.

The dehydration of the droplets to form the gelled spheres can besufficiently complete within 30 seconds, and usually in less than 15seconds, from the time of addition of the precursor material, thataddition being in the form of drops, flowing stream, or by bulk.

The size of the droplets, and consequently the size of the resultantgelled and fired spheres, will be affected by the degree or type ofagitation of the dehydrating liquid as the precursor material is addedthereto. For example, with high shear agitation, e.g. that obtained witha Waring Blendor, relatively tiny droplets (and gelled spheres) can beformed, e.g. with diameters less than 20 microns. In general, gelledspheres with diameters in the range of about 1 to 1000 microns can beproduced.

The gelled, porous, transparent, liquid-filled spheres can be separatedand recovered from the dehydrating liquid in any suitable manner, e.g.by filtration, screening, decanting, and centrifuging, such separationbeing preferably performed soon after completion of the extraction step.Where the gelled spheres are recovered by filtration, filter cakecomprising said spheres and residual dehydrating liquid is obtained. Inany event, the recovered mass of gelled spheres are then sufficientlydried to remove the residual dehydrating liquid and the liquid withinthe spheres, the resultant dried, gelled spheres being convenientlyreferred to herein as green spheres, i.e. dried and unfired. Said dryingcan be accomplished in any suitable manner, care being exercised toprevent too rapid an evaporation in order to minimize fracturing orbursting of the spheres. This drying can be carried out in ambient airand pressure in a partially enclosed vessel at temperatures, forexample, of 20°-25° C. Higher or lower drying temperatures can be usedwith commensurate adjustment of pressure if necessary to preventfracture of the wall of the spheres. During the course of drying, theliquid within the spheres diffuses through the shell or wall of thespheres, as evidenced by microscopic observation of the retreating uppersurface or meniscus of the liquid within the transparent spheres, thusattesting to the porous nature of the gelled spheres. The larger thedried spheres are, the more free-flowing they are. The dried sphereshave sufficient strength to permit subsequent handling. It may bedesired to screen classify them to obtain desired size fractions.

The dried spheres are then fired to convert them to spherical,smooth-surfaced, light weight or low density, rigid, crushable spheres,the shell or wall of which is non-vitreous, synthetic, ceramic,homogeneous, preferably transparent and clear, and comprises metal oxidewhich is polycrystalline or is amorphous metal oxide convertible topolycrystalline metal oxide upon firing at higher temperature. Dependingon the particular oxide precursor material and firing temperature used,the walls of the fired spheres will be porous and heat-sealable orimpermeable, the metal oxide in the walls being present in whole or inpart in the polycrystalline state or in an amorphous state capable ofconversion upon further firing to the polycrystalline state. Forexample, dried, gelled spheres made from Al₂O₃—B₂O₃—SiO₂ precursormaterial can be prefired at 50020 C. to produce porous, transparent,ceramic spheres comprising amorphous Al₂O₃—B₂O₃—SiO₂, which can befurther fired at 100020 C. to form impermeable, transparent, ceramicspheres comprising polycrystalline aluminum borosilicate and anamorphous phase. As another example, dried, gelled spheres made fromTiO₂ precursor material can be prefired at 250°-450° C. to produceporous, transparent, ceramic spheres consisting of polycrystalline TiO₂,and these spheres can be further fired to or at 650° C. to formimpermeable, transparent, ceramic spheres consisting of anatase titania,TiO₂, and even further fired at 800° C. to form impermeable, ceramicspheres consisting of polycrystalline rutile TiO₂. The dried, gelledspheres can be fired in one step directly to impermeable spheres.

The template spheres can be prepared by a spray-drying process. Forinstance, spray-drying solutions can be used that contain nearly anyfilm-forming substance. Spray drying is particularly suited to thepreparation of hollow spheres from solids dispersed in aqueous media.U.S. Pat. Nos. 3,796,777; 3,794,503 and 3,888,957 disclose hollowspheres prepared by spray drying alkali metal silicate solutions thathave been combined with “polysalt ” solutions, and then carefully dryingthe intermediate hollow spheres. The process by which these products aremade must be tightly controlled to minimize the holes, cracks and othersurface imperfections that contribute to porosity that is undesirable inthese products.

In general, largely spherical particles are produced from suchsubstances by forming a solution of the film-forming substance in avolatile solvent and spray drying that solution under conditions thatlead to the production of hollow particles of the size required. Asubstance that breaks down to provide a gas in the interior of theparticle may be required with certain systems to maintain the expansionof the product while it is still plastic and to prevent breakage underatmospheric pressure when the walls have set. Examples of useful blowingagents include inorganic and organic salts of carbonates, nitrites,carbamates, oxalates, formates, benzoates, sulfites and bicarbonatessuch as sodium bicarbonate, ammonium carbonate, magnesium oxalate, etc.Other organic substances are also useful, such as p-hydroxy phenylazide,di-N-nitropiperazines, polymethylene nitrosamines and many others.Selection of a particular blowing agent would be based uponcompatibility with the film-forming system and the intended use of theproduct.

Film-forming systems that are of particular value and which do notrequire the addition of a gas-forming substance as a blowing agent aredisclosed in U.S. Pat. No. 3,796,777, hereby incorporated by reference.Hollow spheres can be produced by forming a homogeneous aqueous solutioncomprising a sodium silicate and a polysalt selected from a groupconsisting of ammonium pentaborate, sodium pentaborate and sodiumhexametaphosphate (other inorganic materials can be used) and then spraydrying the solution under conditions necessary to produce hollow spheresof the size required. The spheres are further dried to reduce the watercontent and to set the walls. In general, the template spheres can havea bulk density of about 0.6 to 20 lbs/ft.³, a true particle density ofabout 2 to 40 lbs/ft.³ and a particle size of about 1 to 500 microns.

The film-forming system in which the organic solvent is used willdetermine the characteristics required, but in general it must be watermiscible and have a boiling point of 100° C. or more. Those solventsused with alkaline systems, such as those containing alkali metalsilicate, must be alkali stable and should not adversely affect thestability of the silicate solution. These characteristics need only befleeting, less than about 3 minutes, as the organic solvent need only beadded immediately before spray drying. In general, those organicsolvents that have a number of hydroxyl groups or exposed oxygens areuseful in the preferred alkali metal silicate polysalt combination.Examples of useful solvents include, among others, cellosolve,cellosolve acetate, ethyl cellosolve, diglyme and tetraglyme. About 0.5to 5.0 parts by weight of the solvent for each 100 pbw of the solids inthe feed solution are required to provide the beneficial effects of theimproved process.

The solution used to form hollow spheres in this method can contain 5 to50% of the film-forming solids. The amount of organic solvent additiveto achieve improved results is between 0.5 and 5%, so that between 0.025and 2.5% of the solution spray dried to form the hollow spheres issolvent. Ammonium pentaborate (APB), sodium pentaborate (SPB) and sodiumhexametaphosphate (SHP) can be used as “polysalts.” If a solution of APBand sodium silicate is used, the total solids would be 5 to 35% with 3to 15% as APB; the ratio of APB solids to sodium silicate solids shouldbe between 0.03:1.0 and 0.5:1.0 and preferably between 0.06:1.0 and0.5:1.0. About 0.015 to 1.75% of the organic solvents would be added tosuch solutions. A system having 0.02 to 0.3 parts by weight (pbw) of SPBper pbw of sodium silicate solids contains 17.4 to 34.5% total solidsand 6 to 7% SPB solids. This combination would require 0.087 to 1.7% ofthe appropriate organic solvent. A system having 1 to 3 pbw of SHP per 1pbw of silicate solids contains 29.6 to 48% of total solids. Thiscombination requires 0.14 to 2.4% of the organic solvent.

The process can be initiated by preparing a solution of the film-formingsolids in water, observing any required restrictions as toconcentration, order of addition, temperature or the like. It isimportant that any restrictions relating to viscosity are observed; ifthe viscosity of the solution is too high when spray dried, fibers mayresult. After the homogeneous solution is prepared, the organic solventis added with rapid agitation to ensure proper dispersion. The resultingmaterial is spray dried prior to any manifestation of instability suchas rising viscosity or gelling. I prefer to spray dry within 10 minutes.

Any conventional spray drying equipment can be used to implement theprocess of this invention. The suspension-solution can be atomized intothe spray tower by either an atomizer wheel or a spray nozzle. Since awide range of film-forming materials and solvents can be used in thisprocess a wide range of spray drying temperatures can be used. Inlettemperatures of 50° to 500° C. can be used with outlet temperatures ofabout 40° to 300° C. Satisfactory products can be prepared from thefilm-forming system of sodium silicate and polysalt by spray drying thematerial at an inlet temperature of 200° to 500° C. and an outlettemperature of 100° to 300° C.

Another method of making the template sphere is by a blowing process orblowing agent process. An example of such a process is forming anaqueous mixture of inorganic primary component and a blowing agent. Themixture is dried and optionally ground to form an expandable precursor.Such a precursor is then fired with activation of the blowing agentbeing controlled such that it is activated within a predeterminedoptimal temperature range. Control of the blowing agent can beaccomplished via a variety of means including appropriate distributionthroughout the precursor, addition of a control agent into theprecursor, or modification of the firing conditions such as oxygendeficient or fuel rich environment, plasma heating, and the like.

In certain embodiments, the precursor for producing the expanded spherecan be produced by combining the primary component, blowing componentand optionally, control agent in an aqueous mixture. This aqueousmixture is then dried to produce an agglomerated precursor. As describedabove, a method of forming a precursor includes the steps of mixing anddrying. The resultant precursor is generally a substantially solidagglomerate mixture of its constituent materials.

The mixing step provides an aqueous dispersion or paste, which is laterdried. Mixing can be performed by any conventional means used to blendceramic powders. Examples of preferred mixing techniques include, butare not limited to, agitated tanks, ball mills, single and twin screwmixers, and attrition mills. Certain mixing aids such as, surfactantsmay be added in the mixing step, as appropriate. Surfactants, forexample, may be used to assist with mixing, suspending and dispersingthe particles.

Drying is typically performed at a temperature in the range of about 30to 600° C. and may occur over a period of up to about 48 hours,depending on the drying technique employed. Any type of dryercustomarily used in industry to dry slurries and pastes may be used.Drying may be performed in a batch process using, for example, astationary dish or container. Alternatively, drying may be performed ina spray dryer, fluid bed dryer, rotary dryer, rotating tray dryer orflash dryer.

The mixture can be dried such that the water content of the resultantagglomerate precursor is less than about 14 wt. %, more preferably lessthan about 10 wt. %, more preferably less than about 5 wt. %, and morepreferably about 3 wt. % or less. It was found that, in certainembodiments, with about 14 wt. % water or more in the precursor, theprecursor tends to burst into fines upon firing. This bursting can becaused by rapid steam explosion in the presence of too much water.Hence, in certain embodiments, the resultant precursor should preferablybe substantially dry, although a small amount of residual moisture maybe present after the solution-based process for its formation. In someembodiments, a small amount of water may help to bind particles in theprecursor together, especially in cases where particles in the precursorare water-reactive.

The dried precursor particles can have an average particle size in therange of about 10 to 1000 microns, more preferably about 30 to 1000microns, more preferably about 40 to 500 microns, and more preferablyabout 50 to 300 microns. The particle size of the precursor will berelated to the particle size of the resultant synthetic hollow templatesphere, although the degree of correspondence will, of course, only beapproximate. If necessary, standard comminuting/sizing/classificationtechniques may be employed to achieve the preferred average particlesize.

Drying can be performed using a spray dryer having an aqueous feed.Various techniques for controlling activation of the blowing agent canbe used such that it is activated at a pre-determined (e.g. optimaltemperature) point in the production process. Such control can beachieved by combining a control agent in the precursor formulation.Another embodiment includes a series of control agents and/or blowingagents such that there is sufficient blowing/expanding gas available atthe optimal temperature. In one embodiment, a series of blowing agentsmay be used which are sequentially activated as temperature rises.

Yet a further embodiment involves distributing the blowing agentthroughout the precursor such that while the precursor is being fired,the blowing agent distributed near the surface is exposed to a hightemperature but the blowing agent near the core of the precursor is“physically ” protected. The thermal conductivity of the formulationcauses a delay between application of heat on the surface of theprecursor to temperature rise within the core of the precursor.Accordingly, blowing agent which is within the core of the precursorwill not be activated until a major portion of the precursor particlehas already reached its optimal temperature. Many blowing agents areactivated by oxidation. Particles within the core of the precursor willnot be exposed to oxygen to the same extent as blowing agent on thesurface, further protecting the blowing agent in the core of theparticle.

Spray dryers are described in a number of standard textbooks (e.g.Industrial Drying Equipment, C. M. van't Land; Handbook of IndustrialDrying 2.sup.nd Edition, Arun S. Mujumbar) and will be well known to theskilled person.

The particle size and particle size distribution can be affected by oneor more of the following parameters in the spray drying process:

inlet slurry pressure and velocity (particle size tends to decrease withincreasing pressure);

design of the atomizer (rotary atomizer, pressure nozzle, two fluidnozzle or the like);

design of the gas inlet nozzle;

volume flow rate and flow pattern of gas; and

slurry viscosity and effective slurry surface tension.

The aqueous slurry feeding the spray dryer can comprise about 25 to 75%w/v solids, such as about 40 to 60% w/v solids.

In addition to the ingredients described above, the aqueous slurry maycontain further processing aids or additives to improve mixing,flowability or droplet formation in the spray dryer. Suitable additivesare well known in the spray drying art. Examples of such additives aresulphonates, glycol ethers, cellulose ethers and the like. These may becontained in the aqueous slurry in an amount ranging from about 0 to 5%w/v.

In the spray drying process, the aqueous slurry is typically pumped toan atomizer at a predetermined pressure and temperature to form slurrydroplets. The atomizer may be one or a combination of the following: anatomizer based on a rotary atomizer (centrifugal atomization), apressure nozzle (hydraulic atomization), or a two-fluid pressure nozzlewherein the slurry is mixed with another fluid (pneumatic atomization).

In order to ensure that the droplets formed are of a proper size, theatomizer may also be subjected to cyclic mechanical or sonic pulses. Theatomization may be performed from the top or from the bottom of thedryer chamber. The hot drying gas may be injected into the dryerco-current or counter-current to the direction of the spraying.

For example, a rotary atomizer has been found to produce a more uniformagglomerate particle size distribution than a pressure nozzle.Furthermore, rotating atomizers allow higher feed rates, suitable forabrasive materials, with negligible blockage or clogging. In someembodiments, a hybrid of known atomizing techniques may be used in orderto achieve agglomerate precursors having the desired characteristics.

The atomized droplets of slurry are dried in the spray dryer for apredetermined residence time. The residence time can affect the averageparticle size, the particle size distribution and the moisture contentof the resultant precursors. The residence time can be controlled togive the various characteristics of the precursor. The residence timecan be controlled by the water content of the slurry, the slurry dropletsize (total surface area), the drying gas inlet temperature and gas flowpattern within the spray dryer, and the particle flow path within thespray dryer. Preferably, the residence time in the spray dryer is in therange of about 0.1 to 10 seconds, although relatively long residencetimes of greater than about 2 seconds are generally more preferred.Preferably, the inlet temperature in the spray dryer is in the range ofabout 300 to 600° C. and the outlet temperature is in the range of about90 to 220° C.

Spray drying advantageously produces precursors having this narrowparticle size distribution. Consequently, synthetic expanded spheresresulting from these precursors can have a similarly narrow particlesize distribution and consistent properties for subsequent use.

The template sphere can be formed by a drop tower process, for instance,forming spheroidal particles from slag and ash. The spheroidal particlesare formed by dropping particles of slag and ash (or other inorganicmaterial) through a heated zone which fuses at least an outer surface ofthe particles. Any type of furnace can be used, such as a drop towerfurnace, a rotary kiln, a fluidized bed, and the like.

The process can involve spherulizing particles of coal slag oragglomerated coal fly ash, resulting from coal combustion (or usinginorganic or volcanic material). The process can include the steps of:

(a) providing a drop tube having an upper portion, a central portion anda lower portion;

(b) delivering a feedstock of particles to the upper portion of the droptube in a manner such that the particles flow in a substantiallyvertical downward path through the feed tube as individualizedparticles;

(c) heating the particles to a sufficient temperature by providing heatto the outer surface of the central portion of the drop tube to cause atleast the outer surface of the particles to melt such that a majority,i.e., at least about 50 weight percent, of the particles becomespheroidal due to surface tension at the outer surface; and

(d) cooling the particles, preferably in the lower portion of the droptube, to prevent agglomeration.

The slag or ash feedstock, which can range in size from, for example,about 0.001 to 10 mm, preferably from about 0.1 to 1 mm, can bedelivered through a feed tube having a discharge port, having one ormore holes, each with a diameter from, for example, at least the maximumparticle diameter of the feedstock, and more preferably, at least one totwenty times the maximum particle diameter of the feedstock, at thelower end thereof.

The template spheres can also be formed by chemical etching, such as byforming a sphere around a bead that is then subsequently removed by heator dissolving. At least 20% by volume of the bead can be removed ordissolved, such as from 20% to 100% or 70% to 99% by volume. The stepsin such methods comprise coating a polystyrene (or other polymer ordissolvable material) bead with an alumoxane solution (or otherinorganic or ceramic material), drying the bead, and then heating thecoated bead to a temperature sufficient to calcine the alumoxane toporous amorphous alumina (or other inorganic material). The coated beadis then washed in a solvent to remove the bead from inside the coating.The remaining shell is then heated to a temperature sufficient to forman α-alumina sphere. Besides alumoxane, other inorganic materials orsolutions can be used.

An alumoxane (A-alumoxane) can be prepared according to the methoddescribed in Chem. Mater. 9 (1997) 2418 by R. L. Callender, C. J.Harlan, N. M. Shapiro, C. D. Jones, D. L. Callahan, M. R. Wiesner, R.Cook, and A. R. Barron, which is incorporated herein by reference.Aqueous solutions of alumoxane can be degassed before use. Dry-formpolystyrene beads, such as those available from Polysciences, Inc., canbe used. Beads of polymers other than polystyrene may be used, so longas the polymer is soluble in a solvent. Likewise, beads of othermaterials may be used, so long as they are soluble in a solvent thatwill not damage the alumoxane coating.

The aqueous solution of A-alumoxane may range from 1-10 weight percent.The aqueous solution of A-alumoxane more preferably ranges from 2-8weight percent, and most preferably is 8 weight percent. Beads 10 mayrange from 1-80 μm in diameter, and are preferably 1-5 μm in diameterand more preferably about 3 μm in diameter.

The solution can be pipetted onto beads and then can be placed in acoated ceramic firing crucible, and allowed to dry in air. The coatingprocess can be conducted in a ceramic firing crucible to minimize theamount of agitation of beads. Beads can be covered or coated one tothree or more times to achieve a uniform alumoxane coating.

The alumoxane-coated polystyrene beads can be fired to 220° C. for 40minutes to burn off organic substituents. The firing converts thealumoxane coating to a porous amorphous alumina coating. This allows asolvent such as toluene to dissolve polystyrene beads but not theamorphous alumina coating. Beads with amorphous alumina coating can bestirred in toluene for 1 hour and then vacuum filtered. Multiple washescan be conducted to remove all of the polystyrene resulting from thedissolution of polystyrene beads, because the polystyrene solution tendsto “gum up ” the surface of α-alumina sphere, precluding removal ofadditional polystyrene. To separate free-standing α-alumina spheres fromany extra alumina resulting from the coating process, the fired (1000°C.) material can be placed in water, centrifuged and filtered. Thecalcination temperature of 220° C. can be used.

A sol-gel process can be used to form the template sphere, such as byforming a metal oxide solution and adding a metal basic carbonate withacid and surface active agent and thickening agent to prepare a sol ofmetal, and then dropping the sol into an alkaline gelation bath and thenrinsing, drying, and calcining. Another method to form a template sphereinvolves the use of a sol-gel-vibrational dropping process, whereinaqueous solutions or sols of metal oxides, such as Hf or Zr, arepre-neutralized with ammonia and then pumped gently through a vibratingnozzle system, where, upon exiting the fluid stream, breaks up intouniform droplets. The surface tension of the droplets molds them intoperfect spheres in which gelation is induced during a short period offree fall. Solidification can be induced by drying, by cooling, or in anammonia, gaseous, or liquid medium through chemical reaction.

The following patents/applications, all incorporated in their entiretyby reference herein, provide examples of the above processes, which canbe adapted to form the template sphere of the present invention: U.S.Pat. Nos. 4,743,545; 4,671,909; 5,225,123; 5,397,759; 5,212,143;4,777,154; 4,303,732; 4,303,731; 4,303,730; 4,303,432; 4,303,431;4,744,831; 4,111,713; 4,349,456; 3,796,777; 3,960,583; 4,420,442;4,421,562; 5,534,348; 3,365,315; 5,750,459; 5,183,493; and U.S. PatentApplication Publication Nos: 2004/0262801; 2003/0180537; 2004/0224155.

The crush strength of the proppant can be determined in a uniaxialloading configuration in a strength testing cell with a cavity diameterof 0.5 inches (12.7 mm). The volume of material admitted to the interiorof the strength testing cell is 1.0 ±0.1 mL. Loading of the strengthtest cell is carried out using a Lloyd Instruments Compression Tester(Model LR30K Plus) at a strain rate of 0.0400 inches per minute (1.016mm per minute). The compressive force (lbf) and deflection in inches arerecorded continuously. The load at “failure ” of the system under testis calculated at a deflection of 0.02 inches (0.508 mm) from the appliedpreload value of 3.00 lbf. The uniaxial crush strength of the system(proppant., template, etc) is quoted in pounds per square inch (PSI) andcalculated from the load applied at 0.02 inches deflection divided bythe cross-sectional area of the strength test cell in square inches.

In one or more embodiments, the template material or sphere can be madeby hybrid methods based on a combination of two or more of theabove-described exemplary methods for making the template material orsphere. For instance, a template sphere having voids can be formed byway of a coaxial blowing process and then a drop tower design can beused to form the spheres. For instance, the drop tower can convert thetemplate material into a sphere or a more spheroidal shape through thedrop tower approach, and/or the drop tower approach can permit a coolingof the template material or sphere.

In the present invention, the present invention provides improvementswith respect to proppant technology. Currently, there is a balance ofproperties that must be met, such as with respect to specific gravity orbuoyancy and sufficient crush strength. In the past, if one wanted toachieve a proppant having sufficient crush strength, the specificgravity and density of the overall proppant was too high such that theproppant would be difficult to pump to the particular location in thesubterranean formation or, when in the subterranean formation, theproppant would not be uniformly distributed since the proppant was tooheavy and would sink in the medium used to transport the proppant. Onthe other hand, some proppants may have sufficient low specific gravity,meaning that the proppant would satisfy buoyancy requirements, however,by doing so, the proppant typically does not have reliable crushstrength and, therefore, the proppant would fail (e.g., fracture orbreak) once in the subterranean formation, if not earlier. The presentinvention achieves the desirable balance of properties by, in at leastsome embodiments, using a template sphere or material which has lowdensity or desirable specific gravities and then strengthening thetemplate by providing a shell around the template sphere therebycreating sufficient crush strength to the overall proppant due to theshell. Thus, in the present invention, at least in one embodiment, thetemplate sphere provides the desirable buoyancy or specific gravityrequirements, and the shell of the present invention provides thedesirable crush strength and related properties. A balance of competingproperties is achievable by the present invention.

The present invention will be further clarified by the followingexamples, which are intended to be exemplary of the present invention.

EXAMPLES

An aqueous acetate-alumoxane solution was prepared according to themethod described in Chem. Mater. 9 (1997) 2418 by R. L. Callender, C. J.Harlan, N. M. Shapiro, C. D. Jones, D. L. Callahan, M. R. Wiesner, R.Cook, and A. R. Barron, incorporated in its entirety by referenceherein. The aqueous solution was degassed before use. The aqueousacetate-alumoxane solution mentioned above, in the solution range of0.5-20 weight percent, was degassed before use. Cenosphere templates inthe size range of 100-600 micron were coated with the alumoxanesolutions as described in the examples below.

In Example 1, spherical polystyrene template beads were coated with thealumoxane solution, ranging from 0.5-20 weight percent alumoxanenanoparticles. The template spheres were submerged in the solution atroom temperature. The solution was then drained, and the spheres placedin a ceramic crucible, which were allowed to dry under controlledconditions. The preferred conditions were at room temperature for 48hours under 70% relative humidity. These dried, coated spheres were thenagitated, and recoated two more times as stated above to achieve auniform coating, and to maximize their sphericity. The spheres were thenheated to 180° C. for 40 minutes to burn off organics and to set thealumina shell. After cooling to room temperature, the spheres werecoated again with the alumoxane solution, dried, and reheated to 180°C., as stated above, which resulted in a thickening of the aluminashell. The templated alumina spheres were then sintered at 1200° C. for1 hour, to convert the phase of alumina in the shell to the crystallinesapphire phase of alumina. FIG. 4 is a SEM image illustrating thespheres formed from the process.

This is a theoretical example. In Example 2, the polystyrene templatespheres can be placed into a container under vacuum, and sufficientalumoxane solution can be injected into the container so as to submergethe template spheres. The container can be vented, followed by drainingof the alumoxane solution, and drying of the spheres under controlledconditions in a ceramic crucible. The preferred conditions can be atroom temperature for 48 hours under 70% relative humidity. The spherescan be recoated according to this vacuum method two more times and driedunder the preferred conditions to achieve a uniform coating, and tomaximize their sphericity. The alumina spheres can be heat processed at180° C., recoated under vacuum, and dried under the preferredconditions, and sintered at 1200° C., as Example 1.

In Example 3, the Cenosphere template spheres were submerged in thealumoxane solution at room temperature. The solution was then drained,and the spheres placed in a ceramic crucible, which were allowed to dryunder controlled conditions. The preferred conditions were at roomtemperature for 48 hours under 70% relative humidity. These dried,coated spheres were then agitated, and recoated two more times as statedabove to achieve a uniform coating, and to maximize their sphericity.The spheres were then heated to 460° C. for an hour to burn off organicsand to set the alumina shell. After cooling to room temperature, thespheres were coated again with the alumoxane solution, dried, andreheated to 180° C., as stated above, which results in a thickening ofthe alumina shell. The templated alumina spheres were then sintered at1200° C. for 1 hour, to convert the phase of alumina in the shell to thecrystalline sapphire phase of alumina. FIG. 5 is a SEM imageillustrating such spheres formed from the process.

This is a theoretical example. In Example 4, the Cenosphere templatespheres can be placed into a container under vacuum, and sufficientalumoxane solution can be injected into the container so as to submergethe template spheres. The container can be vented, followed by drainingof the alumoxane solution, and drying of the spheres under controlledconditions in a ceramic crucible. The preferred conditions can be atroom temperature for 48 hours under 70% relative humidity. The spherescan be recoated according to this vacuum method two more times and driedunder the preferred conditions to achieve a uniform coating, and tomaximize their sphericity. The alumina spheres can be heat processed at460° C., recoated under vacuum, and dried under the preferredconditions, and sintered at 1200° C., as in Example 3.

This is a theoretical example. In Example 5, spherical Styroporetemplates of 300-1200 micron diameter range and 50-200 micron wallthickness can be infiltrated with the alumoxane solution, ranging from0.5-60 weight percent. The resulting diameter and wall thickness of thealumina shells formed can be dictated by the diameter and wall thicknessof the Styropore templates chosen. The template spheres can be submergedin the solution at room temperature. The solution can then be drained,and the spheres placed in a ceramic crucible, which can be allowed todry under controlled conditions. The preferred conditions can be at roomtemperature for 48 hours under 70% relative humidity. These dried,infiltrated spheres can then be heated to at least 180° C., to calcinethe infiltrated alumoxane nanoparticles to alumina, followed by furtherheating at a ramp rate of 0.2° C./min to 230° C. A hold of 1 hour at230° C. can be allowed for burnoff of the Styropore template, resultingin a porous spherical alumina shell. Further heating at a ramp rate of1° C./min to 500° C. resulted in further setting of the alumina shell.The alumina shells can then be cooled to room temperature, andthemselves infiltrated with the alumoxane solution, as stated above forthe Styropore templates. This can result in filling of the void spaceleft by the lost Styropore template. These infiltrated shells can beheated at a ramp rate of 1° C./min to 500° C., to calcine theinfiltrated alumoxane nanoparticles, and to further set the infiltratedalumina shell, followed by cooling to room temperature. These shells canbe infiltrated and calcined once more, to produce a uniform shell ofmaximal sphericity, followed by sintering at 1200° C. for one hour, toconvert the phase of alumina in the shell to the crystalline sapphirephase of alumina.

This is a theoretical example. In Example 6 the Styropore spheretemplates can be placed into a container under vacuum, and sufficientalumoxane solution can be injected into the container so as to submergethe template spheres. The container can be vented, followed by drainingof the alumoxane solution, and drying of the infiltrated Styroporespheres under controlled conditions in a ceramic crucible. The preferredconditions can be at room temperature for 48 hours under 70% relativehumidity. These dried, infiltrated spheres can then be heated to atleast 180° C., to calcine the infiltrated alumoxane nanoparticles toalumina, followed by further heating at a ramp rate of 0.2° C./min to230° C. A hold of 1 hour at 230° C. can be allowed for burnoff of theStyropore template, resulting in a porous spherical alumina shell.Further heating at a ramp rate of 1° C./min to 500° C. can result infurther setting of the alumina shell. The alumina shells can then becooled to room temperature, and themselves infiltrated under the samevacuum conditions with the alumoxane solution, as stated above for theStyropore templates. This can result in filling of the void space leftby the lost Styropore template. These infiltrated shells can be heatedat a ramp rate of 1° C./min to 500° C., to calcine the infiltratedalumoxane nanoparticles, and to further set the infiltrated aluminashell, followed by cooling to room temperature. These shells can beinfiltrated and calcined once more, to produce a uniform shell ofmaximal sphericity, followed by sintering at 1200° C. for one hour, toconvert the phase of alumina in the shell to the crystalline sapphirephase of alumina.

This is a theoretical example. In Example 7, hollow spherical glasstemplate beads of 150-850 micron size range can be coated with thealumoxane solution, ranging from 0.5-20 weight percent. The templatespheres can be submerged in the solution at room temperature. Thesolution can then be drained, and the spheres placed in a ceramiccrucible, which can be allowed to dry under controlled conditions. Thepreferred conditions can be at room temperature for 48 hours under 70%relative humidity. These dried, coated spheres can then be agitated, andrecoated two more times as stated above to achieve a uniform coating,and to maximize their sphericity. The spheres can then be heated at aramp rate of 1° C./min to 460° C., followed by a hold of 40 minutes tobum off organics and to set the alumina shell. After cooling to roomtemperature, the spheres can be coated again with the alumoxanesolution, dried, and reheated to 460° C., as stated above, whichresulted in a thickening of the alumina shell. The templated aluminaspheres can then be sintered at 1200° C. for 6 hours, which resulted inthe formation of an aluminosilicate at the silicα-alumina interface,consisting of mullite and corundum phases. The amorphous silica furthercan serve as a reactive wetting phase to facilitate the resorption ofsome of the alumina, in creating mullite at the interface. The relativeamount of mullite and alumina phases formed can be dependent on theamounts of silica and alumina initially present, and can be calculatedfrom an alumina-silica binary phase diagram. Complete conversion of thesilica phase to mullite can occur in the aluminosilicate sphere, withthe alumina in excess of 60% originally present in the startingmaterial, comprising the alumina phase of the sphere.

This is a theoretical example. In Example 8, a known amount of solid,hollow or porous beads would be fluidized in a fluid bed. An alumoxanesolution would be sprayed into the chamber in order to coat the beads.The beads will then be dried by introducing a heated gas into thechamber or by virtue of their movement through the gaseous “fluid ”.Cycles of spraying and drying can be repeated, depending on thethickness of the coating required. Once the desired thickness has beenachieved, the coated beads are removed and sintered to 1200° C. in orderto convert the alumina to sapphire.

This is a theoretical example. In Example 9, a known amount of solid,hollow or porous beads would be fluidized in a fluid bed. A solution ofpartially cross-linked hybrid alumoxane polymer would be sprayed intothe chamber in order to coat the beads. This would be followed byspraying a curing agent into the chamber in order to set the polymercoating. Alternatively, a molten hybrid alumoxane polymer could besprayed onto the chamber to coat the particles. The beads can then becooled by introducing cooled air into the chamber. In the case of apolymer that requires heating for cure, heated air can be introducedinto the chamber.

In Example 10, 440 mL of water was mixed with 20 mL glacial acetic acid,in which 4 g of Catapal B and 36 g Dispal 11N7-80 boehmites werepeptized with mixing, at room temperature for 2 hours. After sufficientmixing, 150 g of an 8% wt solution of the mixture was spray coated in afluidized bed (Vector fluidized bed, model MFL.01) onto 20 g ofcenospheres, and dried at 80° C., at 130 liters per minute airflow.These coated cenospheres were then sintered at 5° C./min to 500° C., andthen to 1400° C. at 10° C./min, for 2 hours. FIGS. 6-8 illustratesintered microstructures of the above formulation.

In Example 11, 440 mL of water was mixed with 20 mL glacial acetic acid,in which 4 g of Catapal B and 36 g Dispal 11N7-80 boehmites werepeptized with mixing, at room temperature for 2 hours. To this mixturewas added 50 mL of a 1% wt. Fe₂O₃ solution (1% Fe₂O₃ by total solidswt), with additional stirring. After sufficient mixing, 150 g of an 8%wt solution of the mixture was spray coated in a fluidized bed (Vectorfluidized bed, model MFL.01) onto 20 g of cenospheres, and dried at 80°C., at 130 liters per minute airflow. These coated cenospheres were thensintered at 5° C./min to 500° C., and then to 1200° C. at 10° C./min,for 2 hours. FIGS. 9-11 illustrate sintered microstructures of the aboveformulation.

Applicants specifically incorporate the entire contents of all citedreferences in this disclosure. Further, when an amount, concentration,or other value or parameter is given as either a range, preferred range,or a list of upper preferable values and lower preferable values, thisis to be understood as specifically disclosing all ranges formed fromany pair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether ranges are separatelydisclosed. Where a range of numerical values is recited herein, unlessotherwise stated, the range is intended to include the endpointsthereof, and all integers and fractions within the range. It is notintended that the scope of the invention be limited to the specificvalues recited when defining a range.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the present specification andpractice of the present invention disclosed herein. It is intended thatthe present specification and examples be considered as exemplary onlywith a true scope and spirit of the invention being indicated by thefollowing claims and equivalents thereof.

1. A proppant comprising a template sphere having at least one voidwithin the interior of the template sphere and said template spherehaving a Krumbein sphericity of at least about 0.3 and a roundness of atleast about 0.1, said proppant having a Krumbein sphericity of at leastabout 0.5 and a roundness of at least about 0.4, and a continuoussintered shell around the entire outer surface of said template sphere,and wherein said shell comprises a ceramic material or oxide thereof andsaid template sphere comprises a material that withstands sintering at atemperature of 700° C. for 30 minutes, in air or an oxidizingatmosphere.
 2. The proppant of claim 1, wherein said continuous shellhas a thickness of from about 5 microns to 150 microns, and saidtemplate sphere has a specific gravity of 0.01 g/cc to about 1.5 g/cc,and said proppant has a crush strength of about 1,500 psi or greater,and said template sphere has a void volume % of at least 30%.
 3. Theproppant of claim 1, wherein said template sphere having a sphericity ofat least about 0.6, a continuous sintered shell around the entire outersurface of said template sphere, wherein said continuous shell has asubstantially uniform thickness, and wherein said template sphere has aspecific gravity of 0.01 g/cc to about 1.5 g/cc, and said proppant has acrush strength of about 1,500 psi or greater, and said template spherehas a void volume % of at least 30%.
 4. The proppant of claim 2, whereinsaid crush strength is at least 2,500 psi.
 5. The proppant of claim 2,wherein said crush strength is 2,500 psi to 15,000 psi.
 6. The proppantof claim 3, wherein said crush strength is at least 2,500 psi.
 7. Theproppant of claim 3, wherein said crush strength is 2,500 psi to 15,000psi.
 8. The proppant of claim 2, wherein said void volume % is from 30 %to 95 %.
 9. The proppant of claim 2, wherein said void volume % is from50 % to 95 %.
 10. The proppant of claim 2, wherein said void volume % isfrom 60 % to 90%.
 11. The proppant of claim 2, wherein said templatesphere has one central void.
 12. The proppant of claim 2, wherein saidtemplate sphere has one central void and multiple voids throughout saidtemplate sphere.
 13. The proppant of claim 1, wherein said templatesphere comprises a mixture of aluminum oxide and silicon oxide.
 14. Theproppant of claim 1, wherein said shell comprises aluminum oxide,silicon oxide, zirconium oxide, magnesium oxide, or any combinationthereof.
 15. The proppant of claim 1, wherein said template spherecomprises aerogel, xerogel, pumice, envirospheres, perlite, vermiculite,or fired template spheres.
 16. The proppant of claim 1, wherein saidshell comprises two or more layers, wherein one of the layers comprisessaid ceramic material or oxide thereof.
 17. The proppant of claim 16,wherein at least one layer of said shell comprises a resin layer orpolymer layer.
 18. The proppant of claim 17, wherein said resin layer orpolymer layer is the outermost layer comprising said shell.
 19. Theproppant of claim 1, wherein said template sphere has a crush strengthof 100 psi to 1,000 psi and said proppant has a crush strength of atleast 2,500 psi.
 20. The proppant of claim 1, wherein said shellcomprises a reinforcement material.
 21. The proppant of claim 20,wherein said reinforcement material is a fiber, whisker, filler, or anycombination thereof.
 22. The proppant of claim 20, wherein saidreinforcement material is carbon black, fiberglass, carbon fibers,ceramic whiskers, ceramic particulates, metallic particles, or anycombination thereof.
 23. The proppant of claim 1, wherein said shellcomprises a metal carbide, metal nitride, or any combination thereof.24. The proppant of claim 1, wherein said shell comprises a zirconiumoxide, a zirconium carbide, a zirconium nitride, or any combinationthereof.
 25. The proppant of claim 24, wherein said shell furthercomprises magnesium oxide, calcium oxide, cerium oxide, yttrium oxide,scandium oxide, or any combination thereof.
 26. The proppant of claim 1,wherein said shell comprises a metal oxide, a metal carbide, a metalnitride, or any combination thereof, along with silicon, titantium,tungsten, aluminum, boron, or any combination thereof.
 27. The proppantof claim 1, wherein said shell is surface modified with the addition ofsilicon oxide, sodium oxide, potassium oxide, calcium oxide, zirconiumoxide, aluminum oxide, lithium oxide, iron oxide, or any combinationthereof.
 28. The proppant of claim 1, wherein said shell is surfacemodified by applying at least one organic material to said shell. 29.The proppant of claim 1, wherein said shell comprises multiple layers,wherein one of the layers comprises a metal nitride or metal carbide orboth.
 30. The proppant of claim 1, wherein a glazing layer is present onsaid template sphere and in immediate contact with said template sphere.31. The proppant of claim 30, wherein said glazing layer comprisessilica, mullite, cordierite, spodumene, or any combination thereof. 32.The proppant of claim 30, wherein at least a portion of said glazinglayer penetrates or infiltrates below the surface of said templatesphere.
 33. A method of forming the proppant of claim 1, comprisingblowing into template spheres with a coaxial blowing nozzle, asuspension or dispersion comprising inorganic or organic solid particleshaving particle diameters in the range of from 0.001 to 10 microns andheating said template spheres to evaporate any liquid from saidsuspension or dispersion, and then optionally hardening the templatespheres, and further providing a shell around the entire outer surfaceof said template spheres and then sintering said shell to form acontinuous sintered shell.
 34. The method of claim 33, furthercomprising subjecting the template spheres to elevated temperatures tosinter and/or fuse said particles that form the template spheres. 35.The method of claim 33, wherein said suspension or dispersion furthercomprises a binder material, a film stabilizing agent, a dispersingagent, and a continuous aqueous or non-aqueousl liquid phase.
 36. Themethod of claim 33, wherein providing said shell comprises introducingsaid template spheres into a fluidized bed and coating said templatespheres with a suspension or dispersion comprising a ceramic material oroxide thereof.
 37. A method of forming the proppant of claim 1,comprising: a) forming template spheres by tumbling together and mixing(1) solidifiable liquid material comprising a thermally fugitive organicbinder material and a source of void-forming agent adapted to evolve asa gas and convert the liquid material to a template sphere and (2) amass of minute discrete free-flowing inorganic heat-sinterable partingagent particles selected from metals, metalloids, metal oxides and metalsalts that are welled by, and at least partially absorbed into, theliquid material during the tumbling action to form liquid spheres;sufficient parting agent particles being present so that any portion ofliquid material uncovered by parting agent particles tumble againstdiscrete unabsorbed parting agent particles; b) providing conditionsduring the tumbling action, and tumbling for a sufficient time for thevoid-forming agent to evolve as a gas and form a central interior spacewithin the liquid spheres and for the thus-hollowed liquid spheres tosolidify; c) collecting the spheres after they have solidified to ashape-retaining condition; d) optionally firing the hollow spheres tofirst burn out the organic binder, and to then sinter the parting agentparticles to form template spheres; and e) providing a shell around theentire outer surface of said template spheres and then sintering saidshell to form a continuous sintered shell.
 38. The method of claim 37,where in step (d) occurs.
 39. The method of claim 37, wherein providingsaid shell comprises introducing said template spheres into a fluidizedbed and coating said template spheres with a suspension or dispersioncomprising a ceramic material or oxide thereof.
 40. A method of formingthe proppant of claim 1, comprising providing a precursor by forming amixture of inorganic material and a blowing agent in the shapes oftemplate spheres, optionally drying the mixture and firing the precursorto activate the blowing agent, wherein activation of the blowing agentis controlled such that the blowing agent is activated within apredetermined temperature range, and further providing a shell aroundthe entire outer surface of said template spheres and then sinteringsaid shell to form a continuous sintered shell.
 41. The method of claim40, further comprising firing the precursor to activate the blowingagent.
 42. The method of claim 40, wherein providing said shellcomprises introducing said template spheres into a fluidized bed andcoating said template spheres with a suspension or dispersion comprisinga ceramic material or oxide thereof.
 43. A method of forming theproppant of claim 1, comprising: a) delivering particles comprisinginorganic material to an inlet of a furnace; b) passing the particlesthrough the furnace; c) heating the particles as the particles traversethrough a heating portion of the furnace to melt at least an outersurface of the particles such that a majority of the particles becomesubstantially spheroidal; d) cooling the particles as the particlestraverse through a cooling portion of the furnace to deter agglomerationand form template spheres; and e) providing a shell around the entireouter surface of said template sphere and then sintering said shell toform a continuous sintered shell.
 44. The method of claim 43, whereinproviding said shell comprises introducing said template spheres into afluidized bed and coating said template spheres with a suspension ordispersion comprising a ceramic material or oxide thereof.
 45. Themethod of claim 43, wherein said furnace is a drop tower furnace, a droptube furnace, a rotary kiln, a fluidized bed furnace, or a gravity fedfurnace.
 46. The method of claim 43, wherein said passing the particlesin step b) is achieved by gravity.
 47. The method of claim 43, whereinthe step of cooling comprises cooling the particles as the particlestraverse through a cooling portion of the furnace, wherein, aftercooling, the particles have a substantially higher bulk density at roomtemperature than prior to delivering the particles to the inlet of thefurnace.
 48. A method of forming the proppant of claim 1, comprising: a)coating dissolvable beads with a solution comprising an inorganicmaterial; b) drying the beads so as to form an inorganic coating on thebeads; c) heating the beads to a first temperature, wherein the firsttemperature is sufficient to form a continuous inorganic coating and isnot sufficient to decompose the beads; d) dissolving all or some portionof the beads; e) optionally removing the dissolved beads from within thecontinuous inorganic coating; and f) optionally heating the continuousinorganic coating to a second temperature that is sufficient to formsaid template spheres, and g) providing a shell around the entire outersurface of said template sphere and then sintering said shell to form acontinuous sintered shell.
 49. The method of claim 48, wherein step e)and step f) occur.
 50. The method of claim 48, wherein said dissolvablebeads are polymeric beads.
 51. The method of claim 48, wherein saiddissolvable beads are inorganic beads.
 52. The method of claim 48,wherein said dissolvable beads are geopolymers, ceramic beads, metallicbeads, glass beads, ground shells, rust, diatomaceous earth, diatomite,kesselgur, fly ash, gypsum, spent catalyst, clinker, blast furnace slag,or any combination thereof.
 53. The method of claim 52, wherein at least20 volume percent of the dissolvable beads are removed in each templatesphere.
 54. The method of claim 48, wherein said dissolvable beads areplant seeds; plant husks; soil; crushed nuts; ground hulls of nuts;whole nuts; plant pips; cells; coffee grinds; food products; algae;plankton; animal eggs; wax; surfactant-derived liquid beads; powdered,ground, or crushed agglomerates of wood products; powdered, ground, orcrushed whey; cellulose; soap; bacteria; rubber; powdered milk; animalwaste; unprocessed polymeric resin; or animal hair.
 55. The method ofclaim 48, wherein said dissolvable bead is spheronized prior to saidcoating of step a).
 56. The method of claim 48, wherein providing saidshell comprises introducing said template spheres into a fluidized bedand coating said template spheres with a suspension or dispersioncomprising a ceramic material or oxide thereof.
 57. A method of formingthe proppant of claim 1, comprising: a) coating burnable beads with asolution comprising an inorganic material; b) drying the burnable beadsso as to form an inorganic coating on the beads; c) heating the coatedburnable beads in controlled fashion such that the burnable beadundergoes controlled thermolysis leaving an intact substantially hollowtemplate sphere; d) providing a shell around the entire outer surface ofsaid template sphere and then sintering said shell to form a continuoussintered shell.
 58. The method of claim 57, wherein said beads arepolymeric beads.
 59. The method of claim 57, wherein said beads areplant seeds; plant husks; soils; crushed nuts; ground hulls of nuts;whole nuts; plant pips; cells; coffee grinds; food products; algae;plankton; animal eggs; wax; surfactant-derived liquid beads; powdered,ground, or crushed agglomerates of wood products; powdered, ground, orcrushed whey; cellulose; soap; bacteria; rubber; powdered milk; animalwaste; unprocessed polymeric resin; or animal hair.
 60. The method ofclaim 57, wherein said burnable bead is spheronized prior to saidcoating of step a) by spray drying, rolling, tumbling, or otherprocesses.
 61. The method of claim 57, wherein providing said shellcomprises introducing said template spheres into a fluidized bed andcoating said template spheres with a suspension or dispersion comprisinga ceramic material or oxide thereof.
 62. A method of forming theproppant of claim 1, comprising forming a template sphere and providinga shell around the entire outer surface of said template sphere, andthen sintering said shell to form a continuous sintered shell.
 63. Themethod of claim 62, wherein said sintering comprises liquid phasesintering, reactive phase sintering, solid state sintering, orpressure-assisted sintering.
 64. The method of claim 63, wherein saidpressure-assisted sintering comprises the application of external gaspressure during heat treatment, with pressures ranging from ambient to1500 PSIG.
 65. The method of claim 62, wherein said sintering comprisesindirect radiant heating, direct infrared radiation, direct conductionof heat flux from an environment to said proppant, excitation ofmolecules of said shell, and consequent heating of said shell byelectromagnetic radiation, or inductive coupling of the shell to anexternal excitation field of alternating current.
 66. The method ofclaim 62, wherein said forming of said template sphere is by a spraydrying process, a dehydrating gel process, a sol-gel process, asol-gel-vibrational dropping process, a drop tower process, a fluidizedbed process, a coaxial nozzle gas-bubble process, a thermolysis process,a chemical etching process, or a blowing process.
 67. The method ofclaim 62, wherein said template sphere has a template surface withcracks or flaws, and said method further comprises surface repairingsaid cracks or flaws with a composition comprising at least oneinorganic or ceramic-containing material.
 68. The method of claim 67,wherein surface repairing comprises infiltrating said cracks or flawswith a suspension comprising alumoxane, mullite, or a combinationthereof.
 69. The method of claim 62, wherein said proppant has aproppant surface with cracks or flaws, and said method further comprisessurface repairing said cracks or flaws with a composition comprising atleast one inorganic or ceramic-containing material.
 70. The method ofclaim 69, wherein surface repairing comprises infiltrating said cracksor flaws with a suspension comprising alumoxane, mullite, or acombination thereof.
 71. The proppant of claim 1, wherein said proppanthas each of the following characteristics: (a) an overall diameter offrom about 90 microns to about 1,600 microns; (b) spherical; (c) saidshell is substantially non-porous; (d) said proppant has a crushstrength of about 3,000 psi or greater; (e) said coating has a wallthickness of from about 15 to about 120 microns; and (f) said proppanthas a specific gravity of from about 0.9 to about 1.5 g/cc.
 72. Aproppant formulation comprising the proppant of claim 1 and a carrier.73. A method to prop open subterranean formation fractions comprisingintroducing the proppant formulation of claim 72 into said subterraneanformation.
 74. A method of treating a subterranean producing zonepenetrated by a well bore comprising the steps of: (a) preparing orproviding a treating fluid that comprises a hydrocarbon or water carrierfluid having the proppant of claim 1 suspended therein, and (b) pumpingsaid treating fluid into said subterranean producing zone whereby saidparticles are deposited therein.
 75. The method of claim 74, whereinsaid treating fluid is a fracturing fluid and said particles aredeposited in fractures formed in said subterranean producing zone. 76.The method of claim 74, wherein said treating fluid is a gravel packingfluid and said particles are deposited in said well bore adjacent tosaid subterranean producing zone.